In my experience, up to 8 with pathfinder or 3e, up to 10 with AD&D, no more than 7 for 4e.
If you had asked "preferred"? No more than 5, for ANY RPG.
I have "managed" fifteen players several times, but it was too much work.I voted 8, I felt that when I had 8 it was basically straining my limits to wrangle them all to pay attention and I had to implement additional steps in order to do so, the steps worked, but it was much more work for me.
As of 46 votes the results have a strange bell-curve. 5 and 6 have a high percent, as does 8. But 7 represents a strange lull in perceived maximum. Is the thought here "well, if I'm going above 6 I may as well jump all the way to 8" ?
As of 46 votes the results have a strange bell-curve. 5 and 6 have a high percent, as does 8. But 7 represents a strange lull in perceived maximum. Is the thought here "well, if I'm going above 6 I may as well jump all the way to 8" ?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.