LARP-running has any number of challenges that tabletop gaming can't reasonably face. The specific case I want to talk about today is module parties. I assume modules aren't a significant part of salon LARPs, though of the three salon LARPs I've played, two included adventure sequences outside of the main area of gameplay. Boffer LARPs very commonly include modules, and one of the constant questions is "how many people can go?" Modules are often rich with plot developments, action, and treasure - the exciting stuff. On an in-play level, there's also often more danger and narrative importance - which means that in character, it makes sense to want to reduce risk.
Reducing risk is good character decision-making, but awful for entertainment value. Reduce it enough, and there will be people just standing around - they couldn't have known this ahead of time, but they might as well have stayed in town. In general, you don't want players to have to make choices based on good gameplay for themselves or others, though a certain sensitivity to leaving room for others to have fun also is an important element of sportsmanship. As far as that goes, failing to reduce risk when you can is a good way to invite a messy death in for afternoon tea.
That means that it's squarely on Plot to figure out why only so many people can go, and not more. Oh, one more than the module runner planned for isn't a problem, but (x+3) is a threshold of a problem, and beyond that it's all but certain that someone will be twiddling his thumbs. Working on limited resources, a module runner can't possibly entertain everyone - there's a time and place for modules that include the whole playerbase, but that's an entirely separate matter. Players usually understand when not everyone can go along, though very large teams often run into the problem of having to ask several team members to stay behind. (Very large player teams, or "mega-teams," are yet again another conversation. They do have their up-sides, but there are a lot of inherent challenges.)
These ideas are a primer for beginning staffers or a recap for the experienced, not master-class material. If you're an experienced LARPer and LARP-runner, I'm not dropping any truth bombs here, but maybe laying out the basics will trigger a new idea for someone, or one of my enterprising and good-looking readers (who are at least 10% more attractive to members of the gender(s) they prefer, if any, than people who do not read this blog) will add more ideas in the comments.
There are a lot of good answers to this, and at some point or other a Plot committee will probably use most of them. The Wildlands campaign (and from what I dimly understand maybe also Madrigal?) include indestructibly-powerful monsters that only threaten groups of people in excess of their universal party size requirement. In Wildlands, x is always 7. I've mentioned banshees as a field battle mechanic; ultimately the banshee is a key part of why Wildlands works.
Banshees aren't for every game, but having an easy and well-known reason for limiting module party groups can make life a lot easier as long as the ramifications of the idea don't break a bunch of other things. So let me trot out a few ideas - some for intermittent use, some to be a well you can always go back to.
If your in-game location is on the water - and even better if the setting is an archipelago - a rowboat or small sailing vessel (which doesn't need to appear on-camera ever) can easily be described as only carrying x people safely at a time. If you need to go with a smaller number than usual, just make the PCs bring along a few massive haversacks full of gear.
Other vehicles work too, if your setting makes them feasible. Horse-drawn vehicles presuppose horses near the game setting - something a lot of games, including DtD avoid, since it's not like we can ever put horses on-camera. Ox-drawn carts are slow enough that someone could outpace them on foot without much problem.
Eclipse used the back of +Garrick Andrus's truck on a few occasions, and the back of a U-Haul truck at other times. Sometimes this limited party size, and sometimes it just forced the first team in and last team out to establish and hold beachheads while waiting for the rest of the team, or evac.
If your fiction does allow horses near the game area, to hell with the carriage or whatever. There are x-many horses available for this trip.
Travel through a gate, rift, or teleportation effect. Different limitation this time than last time? It's magic, hell with it. I believe in giving magic a system as much as anyone and more than most, but leaving some wiggle room for "the winds of magic are a little different today" isn't the worst thing in the world. Unless you're the kind of game where it's all good for teleportation to be super common (nothing wrong with being so, but it's not for every setting), you'll want to use this sparingly, for the plotlines that are very magic-focused.
When Tracking an enemy, it's uncommonly useful to keep party sizes small - after all, they're probably chasing an individual or a small group. Also, in my experience most Tracking modules are unplanned, emergent events that stage a chase through the woods because an NPC did a runner, and the PCs want a second chance at him.
"They'll hear a large party coming," or the variant "they'll be scared off by your smell," are classics that are especially useful for modules that amount to a cabin full of NPCs out in the woods. It's a bit shaky as far as reasoning, but PCs owe it to Plot to meet them halfway and not spend too much time questioning it.
I would be intrigued to see a unique magic item, probably times-ever or times-per-event item, that allowed its bearer to be an And Guest for specific module justifications. Not so you can gatecrash another team's module (so the item only works if one or more people on the module do a Thing that allows the item to work in the first place), but so PCs can sometimes reply to the module hook's "no you may not" with a "yes I may." This meshes exceptionally well with a banshee mechanic, but also with #3, above. The point here is that the players get a tiny bit of risk-management control.
We're in a Flaming Rush is the version where the module hook shows up in town, approaches a group that coincidentally is close enough to x to satisfy the module-runner, and press-gangs them into adventure. This is immensely useful if handled well, but everyone who has ever hooked a module knows that the slightest delay ruins it. If someone needs to run back to their cabin for this one thing, don't wait for them. Don't wait long enough for one more person to walk into the building, or you're sunk.
The door to the Campbellian heroic underworld admits exactly one user per key. We have exactly x keys, isn't that lucky? Or whatever other single-use tchotchke you may have available to hand out in the exposition step of this plotline. This is a particularly useful explanation because it brooks so very little argument, and because it hands out the key props as lovely parting gifts.
A master-class variant of this puts some step of key production or acquisition in the PCs' hands. Let's say the module concept could reasonably support up to x players, where x is really absolutely the most you could bear to support. Something the PCs do lets them make keys, at an exponentially increasing cost per key.
So, I've never played an Accelerant LARP, but reading their rules, it's immediately clear that Social Modules are a thing. Invitations are the non-magical version of this idea, and they fit even better with the idea listed above in 5.1. "This magic item lets me be your date" is very funny to me.
As a more general version of 7.2, any time you're going on the module because you've been invited to visit, the host can politely request no more than x guests. The improved version of this is to mess with the party composition - for example, Dust to Dust could send invitations to five wizards, with permission that they should each bring one homunculus. Or five alchemists, each to bring one test subject drinking buddy. Good times.
All of this gets is complicated further by repeater modules that are unified by a single premise and have to be completed multiple times to move the plot forward. If you're not familiar with these, the idea is that the module-runner is going to get multiple uses out of the same writeup and setup by sending multiple PC teams through it - and hey, you've already got the monsters in-place and briefed too! It's great as long as you can keep the justification for the repetition fresh. Personally I prefer it if repeaters are used rarely enough that they're not an expected part of a weekend's formula. The gold-star, A++ version of this is if you can afford (staff- and NPC-budget-wise) to run two repeater modules simultaneously, and force PCs to recombine teams in unusual configurations. There are a lot of worthwhile points to be made about best-practices repeater modules; one of DtD's best experiences was a dual-repeater situation where both modules had brutally short time limits - I think it was five minutes for one and ten for the other? Something like that.
Anyway, the point is that repeater modules can be a supremely efficient means of creating and staging content. Overuse violates the illusion of the game in a way I can't be comfortable with, but not using them at all is leaving money on the table. But what does this have to do with the price of tea in China?
Once a team is together and knows they can handle the dangers, why wouldn't they try to go a second time? Totally setting aside issues of feeling like it's "their" plotline and they want to engage with it even more, they know what to expect. Obviously, this is Not Kosher. For one, despite their desire to solve the problem and save the day, they're going to be bored facing the same challenge again. For another, the point of the repeater module is getting a substantial percentage of the playerbase to see the content. Sure, they want the excitement of focused attention, in the form of a module - it's nice to want things. So how do you explain this, if they've already satisfied the requirements to go on the module? This is a lot harder to do without overt magic or other supernatural business.
(As before, these are some of the more common ideas. Got more I haven't included? The comments field is intrigued by your ideas and would like to subscribe to your newsletter.)
The most obvious reason is that there's a magical door you can't pass through twice. There's one or more doors leading out, but the door won't let someone who has passed it once pass it a second time.
There are about a million different ways to phrase this magical door you can't pass through twice. The gods may smite you;
the Dark Powers may Mark thy soul;
the gatekeeper may turn you away at the door.
The step down from "Absolutely No" is still good enough in most cases, and has the side benefit of leaving you some wiggle room if, for some reason, the last team of the day can't complete a run without someone who's already gone. (This happens only if you're insanely efficient and manage to get your whole playerbase through a module... ideally without them feeling rushed.) This is the stacking debuff option, where people who go through a door/across a bridge/whatever are afflicted in a small way. Going twice doubles (or more) the affliction. It's okay, probably better really, to start the debuff stack on the second trip, but in any case the players I know get the hint and back off.
A non-magical variant of the debuff is the idea that monsters know your scent or have in some way learned from your presence there before, and thus attack you all the more fiercely or know how to counter your preferred moves. This runs the risk of sounding more like a fun challenge than a reason not to go, so make sure you know what motivates your players before trying his one.
It's conceivable to have module goals where the players just want to get a particular kind of object found in the module area. For some magical reason, that treasure is bind-on-pickup and you can only own one at a time.
One neat "widget" to hand out is a one-shot non-stacking buff - this goes a little further toward motivating players to get everyone through that module, as long as there's enough feeling of external threat in the setting that players want to strengthen each other to ensure mutual survival. Particularly fun if this buff opens the way to the Saturday night climactic encounter. It's also nice for being a solution that is more carrot than stick.
For social modules, the "repeater module" might be an audience with the Duke, just done a few at a time and with some additional complicating features. Of course you can't go back in - you've said all that need be said.
For another non-magical justification, I've seen games explain that the multiple entrances to these "different" places that the parties are going are separated by long distances. This involves some hokum on the passage of time, but I'm from a community where players are super forgiving about that.
My eternal favorite, though, is also non-magical: by political agreement. In a lot of settings, the main game location is populated only by the political cooperation of multiple states and organizations. One of the things they agree to is that they'll divide any really interesting stuff they find - which means they have to share around the exploration rights. We've implemented this approach in DtD less definitively than intended, but it still comes up every now and again.
So that's what I've got off the top of my head. I hope that this provides some useful jumping-off points for your own LARP-runner's bag o' tricks. It's funny - there are heaps of blogs with lists of ideas like these for tabletop GMing, but if I've ever seen a comparable list for LARP usage I can't think of it. There are vanishingly few tabletop situations where any of these would be useful for the reasons they're used here. The few boffer LARP blogs I know don't focus on the Plot and module-level practical advice, though there's some great stuff out there for the logistics and other fundamentals, just as there's some great stuff for high-level discussion of player comfort levels and what kinds of stories we tell.