Assuming they all turn up both games I play in/run are 6 players and a GM (different people for the most part). Theory and practice are quite different though, especially
with players who work shifts.
Fortunately the 9-12 player, 16-hour straight Shadowrun games while drinking heavily are behind me, as is sleep deprivation, sugar rush from too much red bull and insanity from replying to the dumb questions that many troublemakers can come up with while sleep deprived. Six is nice and manageable.
For D&D four just seems to small, feels like you can’t cover all the bases, perfect for Ars Magica though.