No matter how big a game gets there is always around 3-4 that really carries the weight of the game. They will be the ones the most active and the most proactive with their characters while the others will come now and again if at all.
What the lady above said is very true. There are always a few players that are really the core of the game then comes the next set of popular players and then on the outer rim you have the ones that show up every now and again.
I think that's up to the Dungeon Master, every Dungeon Master is different. There shouldn't be a number that is outside the comfort level of the Dungeon Master because its a game to be enjoyed and we often forget that the Dungeon Master is playing the game as well we aren't giving him a 9-5 job that should feel like a burden.
Yeah I think somewhere between 5-8 is probably optimum. I've played with fewer and with more. I think more is better than fewer but 5-8 is my guess on the 'best' number.
KN,
That game with 10 must have been a bear for the Dungeon Master. That is very impressive.