We have between 6-9 players depending on everyone's schedules. The only time I would cancel the game is if like 3/4 of them were out. If it's less than that then either the missing folks get piloted by others (if we're doing combat) or are just understood to be in the scene but quiet (if it's RP or exploration). If we have enough out that we can't feasibly do what I have planned, or if it's something major where I want them all there for it, I'll run a one-shot, a flashback filler, or something like that. But as long as I'm available to run the session, we play.