For me, 3-4 players is optimal. 2 is too small a group, IMO to handle everything that I would want to throw at them, and 5-6 is doable. More than that, and there's too many folks around to have any actual roleplaying, it seems, plus it's too easy to have characters get lost in the crowd. Obviously, I prefer a more roleplaying type of environment rather than big groups of explorers/dungeoncrawlers, in which case I'd probably revise that number up fairly easily.
Currently I have 3.