I answered 3, on the assumption that you were asking about number of *players* and not PCs. I usually have three, lately four, sometimes two, and one guy who wants some solo time--but I think there are something like twelve PCs between the three (four) of them. Only one PC per player is onscreen per session though.
I don't enjoy groups with more than four players, and frankly even four is pushing it. Two or three is ideal--even if they split the party you can still give enough attention to each player to keep things moving. Or rather, each player still has ample opportunity to perform the describe/decide/declare/resolve loop which is the essence of D&D. I've played as a player in larger groups (10-ish) and it was like herding cats--took forever to get any decisions made, and there wasn't any realistic possibility of a given player getting to do individual actions (i.e. splitting the party), which is why individuals were prisoner to the needs of the group, which is part of why decisions took so long to make.