It depends
As always, it depends.
Mystery type stuff, or political stuff, I prefer a smaller number (more than 3 for mysteries and suspension of disbelief goes bye-bye when they start questioning people as a group).
For dungeon crawls and pick-up games a larger bunch is nice, 'cause you're not stymied by sudden absences.
Current group is me plus six which is about at my limit. I've played in a game with 10 players plus GM. It's probably not fair to compare that with the others, because the only session where we had 10 was mostly just a big fight scene.