Party Size in D&D

How big is too big for a gaming group? How small is too small?

D&D (in particular) is set up to run with 4 players and a GM. However, you can run games with significantly more players or significantly less. At a minimum to run a game, one player and one GM. Any less than that and you're not running a game, you're writing a short story/novel. As long as you have at least one other person, you can run a game.

There are special considerations when running with less than the "expected" number of players. If the experience is handed out for combat, you may need to alter the encounters for combat, since there aren't enough players. With fewer players experience will need to be handed out for more than just combat (or increase the xp awards for combat). Roleplaying would be the easiest method for awarding extra XP. Also, running a game that isn't focused on combat allows for much smaller parties than a combat centric game.

On the other end, there really isn't an upper limit to how many people can playing a game. I've played in games that had 12 people in them. The only limit to how many people can play in a single game is determined by two things: how much effort the GM can put into it and the patience of the players. Certain GMs can run more, other can run less. GMs also have the option to co-GM if the group gets too big. However, if the players aren't willing to put up with a group that size, it won't work. The more people you add to to a group, the length of time everything takes increases exponentially.