This is the second part of this post, and covers LARPs (and MMORPGs) specifically. Read the earlier post first.
LARPs are a more complex situation than either tabletops or CRPGs. They (and MMORPGs) have a weird feature:
Basically, it doesn't matter how many people you have in your LARP. It only really matters how many people a person will interact with - I've been perfectly happy playing 40-person LARPs where I only interact with five or six people. From my point of view, they would have been just as much fun as five or six person LARPs.
So, you're not asking for a total population, you're asking for a level of interaction. Some games will have infrastructures which require a certain population to support specific levels of interactions, though. For example, you can only buy potions from someone if there are enough people making potions that there are potions available to buy.
That's kind of a slightly more advanced topic. Let's try to stick to basics.
The density of these games is kind of an interesting question. You can raise and lower it all you like, but there is going to be a natural tendency for people to inject socialization into the game. But different people prefer different amounts of socialization. Some prefer a lot, some prefer almost none.
What this means is that you want to include long-term density rather than short-term density. Short term density is things like skills and stats and math. Long-term density is things like items, politics, and plot.
If you put in mostly long-term density, then it will merge with role play smoothly - some people will role play more, some people less, but nobody will feel left out. The real difficulty with that is that some people will probably feel they are doing poorly and want to quit. You can solve most of that problem with judicious use of tangental goals, rather than competitive goals.
However, when you get right down to it, these games are real-time. That means the density is going to be chopped down - or bulked up - depending almost entirely on the speed of the game.
Most LARPs and MMORPGs are very slow, so their density is very high. However, in LARPs where the players are almost always pressed for time the density drops down quite low - to the point where even the simplest little bit of complexity seems like a profound plot twist, just because nobody has really had time to think about it. People will actively just drop complex parts of the game because they simply don't have time.
It's simple: speed and density oppose each other. They seek equilibrium. Most LARPs choose to favor density. Nothing wrong with that.
Here's the detail: the interconnectedness of a LARP or MMORPG sets where that equilibrium is. Interconnectedness is how many people a player will be significantly involved with over the course of a session (or, in most LARPs, the whole LARP). Most LARPs run with it being 5-6. MMORPGs seem to run slightly fewer.
The more interconnected a game is, the lower the equilibrium. The less of both speed and density you can have. The less interconnected, the higher the equilibrium. The more of both speed and density you require to maintain interest.
This is because, of course, interconnectivity is a kind of density.
Anyway, here's the basic idea: interconnectivity + density + speed = 1
You probably know how to raise speed. Simpler resolution is the easiest way, but don't forget that forcing players to move at the speed you set is very important. So timed events, forced challenges, revealed data, and other things which force the player to move also increase speed. (They can also increase density and interconnectivity, so be careful.)
If you want to raise density, try to raise long-term density rather than short-term density. Long-term density can be safely ignored by people pressed for time, with a preference for RPing, or without the brains to manage it. Short-term density is forced on everyone, so it's not nearly as adaptive. If you rely heavily on long-term density, you can change the equation from an "=" sign to a ">=" sign.
Raising and lowering interconnectivity is probably the least natural part of the equation. Most games you've designed and/or played probably operated on total interconnectivity. Every character knows and interacts with every other character. It is very rare that there are more than eight of them.
In a LARP and an MMORPG, this is usually impossible. Instead, you have to change how likely a player is to work with or against someone.
In a LARP, this is usually done in the writing. Plot hooks, "you know this guy"s, hunts for widgets someone else has... all of these things and many other basic elements of LARPy plot increase interconnectivity.
In an MMOPRG (and some LARPs) it is done primarily through infrastructure and resources. You need a potion? You have to find someone to sell you a potion.
The problem with this is that it typically leads to very shallow interactions. To force deeper interaction, most MMORPGs essentially force you to form parties. That is a decent way to do it.
The big difference to keep in mind between an MMORPG and a LARP is that a LARP has consistant characters, whereas an MMORPG has unreliable characters. In a LARP, it's unlikely any given character is going to leave and log out. This leads to cliquing (clikking?). People will form into teams and stay in their teams. This will usually lead to "congealing" - everyone ends up in one or two packs, and stomp around the game in unison.
There are a couple of ways to break this up. First, you can use some speed-enhancing tactics to force them to split up the group to acheive two or more simultaneous goals. Second, you can use antisocial, megalomaniacal, and opinionated characters who act as sandpaper and refuse to work with specific other characters.
But congealing isn't necessarily bad. It does tend to radically unbalance the game, and decrease the amount of agency any given player has, but it provides a sense of timing and momentum that no other situation can emulate.
My suggestion? Fragment the groups in the early game, then let them congeal in the late game. It's pretty easy to direct, in the late game, and that's when everything SHOULD be coming to a head.
Heh. This was kind of babbly, sorry. But I hope it was clear.