It wasn't supposed to happen like that because the plan was that I would be running in a small backroom in Elton's Lifetime Loser Lounge (a local game-ish store) for three or four people. My brother, Poot, was setting things up for me since I was pulling ridiculous hours at my job. He had reserved the room and let two of our friends know about the game. So you can imagine my surprise when I walked in that evening and found out that we would have to be playing in the big room.
Naturally, I figured, the other rooms must already have groups playing in them, but as I walked down the hallway I was the only person in the back rooms. I started setting up my gear and in came Poot with the two players I had planned on. As they set down and started rolling up their characters more people kept coming in and asking me how we were rolling stats - and like a damned fool I told them! Not once did it occur to me that I should say, Listen this is a closed game. It's just going to be the four of us. Instead I just kept answering their questions and making notes in my book hoping that it wouldn't sprawl into the sort of massive cluster-fuck that would keep everyone from ever playing with me again.
The table filled up, everyone was done making their characters, and they were all looking at me. I looked back at them and thought, Well, I'm fucked, and started running.
When you read about running for large groups one of the first things you'll notice is that online most everyone talks about tables with eight to twelve players and its generally spoken of in the same terms that you use for car crashes and derailed trains. People like to see the aftermath and comment on what should have been done but nobody wants to be involved.
Why is that?
According to the conventional wisdom I've read Dungeons and Dragons players are as difficult to plan for as it is to herd cats. Each player acts on their self interest first and the group's second. Whether you've prepared a detailed dungeon crawl, a hex-ploration, or are coming up with an intricate and convoluted plot that would make Tolstoy proud the end result is going to be the same: your players will not do anything you've prepared for and all your efforts are wasted as they act on their self-interest first. When you expand the group beyond the comforts of the four to six player range these problems are exacerbated and you have set up a situation where things will spiral out of control; inevitably the session will crash and burn.
The problem with this line of thinking is that it's bullshit when applied to larger groups.
The dynamic in how players act changes because they develop these amorphous cliques within the large group. Each session an individual within one of these mini-groups will take center stage and drive his team towards a goal of his own that aligns with theirs, but they will drop him and shift to another leader if this goal begins to dominate them or starts to run counter to their own individual goals. Typically these circles range from three to six players but I've seen occasions when they will grow up to twelve players before slowly breaking up again. For a Dungeon Master running a large group recognizing the development of these cliques and helping funnel them towards their desires is how you keep a session from crashing and burning.
Yet allowing the groups to form into these little mini-parties isn't enough; you have to control the clock. In a group with fifteen players it is incredibly easy to let one player dominate the action (it's even easier in smaller groups); so how do you combat that and prevent the players not involved from becoming bored and disruptive?
Some people want to limit the actions of each player to a set time, say three minutes, and keep the group on a constant rotation. That's part of the right answer but if you do it on an individual basis then you're doomed to failure. Think of of it this way: in that first session if I had limited each player to three minutes of action before I moved on to the next player then forty-five minutes will have passed before I could come back to that first player and let them act again. That's a terrible way to run.
I've seen others who choose to break up the larger group into smaller, static ones. The idea is that you break the larger group into easy to handle portions that tend to work well together and always ask them to announce their actions together. The problem is that this method will often, though not always, make people resistant to doing anything together. They'll feel like you're robbing them of their choice in the matter - even if you're just breaking them up into the cliques they're already forming.
My solution is to work my way around the table from left to right asking each player in turn, What are you doing? After getting a response I ask, Does anyone want to go with X? and divide the group along those lines. Then I have everyone going with the first player tell me what they'd like to do and I run a short (10 to 15 min) series with them before I move on to the next player who didn't act and repeat. Once I have a couple of these groups going I move them to a point where I can jump to the other group and still keep them interested in what's going on.
How do I do that?
I usually jump between groups right before they interact with a major non-player character, or when they've heard the click of a trap being sprung, or when they realize they're about to be attacked by a patrol of orcs. The event that I jump between is typically something the players want to be involved in or that builds a sense of tension for them so that I can keep them thinking about what's going on as I deal with the others. It can take some getting used to but once you've gotten a handle on jumping like this in a game it becomes a really easy way for keeping everyone involved and the game constantly moving forward.
Now before I leave you thinking that you could use this method to run for thirty or fifty players let me advise you that there is a limit to how many players you can handle with a single Dungeon Master, and that number will vary depending on the individuals involved. For me that number is a solid 25, though any group with over 20 tends to become cumbersome unless you've got a lot of players who work really well together.