I know this question is as much about circumstances as it is gameplay, but what is the ideal player party size?
When I was in college, it was the more the merrier, but gameplay bogged down when I went above 5 or 6 players. I used to run AD&D 2e sessions on Friday mornings back in those days, and I could have up to 10 players to juggle. As a DM I felt I was giving less then my best performance.
My old gaming group ranged from 3-6 players for the most part... although there were times that got huge too.
Or does it depend upon game system? D&D thru 3.5e has 4 core classes, and with multi-classing, more then one role could be fulfilled per PC. So, you didn't need 4 PCs, but it did help for balance.
Tunnels & Trolls has 3 core classes: Warrior, Rogue and Wizard. Rogue isn't a D&D Rogue, but more of a D&D Fighter / Magic-User with possibly some thief thrown in. I'm not sure what the ideal party size would be for a T&T game, but I suspect one of each core would fill the rolls nicely.
What is your ideal player party size when you GM a game?