I believe that a DM can handle up to nine characters, so long as the game is PbP.
However, nine is not optimal. Someone (either creamsteak or garyh) pointed out that 4 is best for RPing, and 6 is best for combat (or 4 is best for combat, and 6 is best for RPing... one of those two).
IMO, it depends upon the game. I'm allowing six-eight people at a time in the Psionicle. The Iconics have a large number, of course. The Revolution! game was going to have nine, but has now somewhere in the five or six range.
It depends upon the game and the skill of the DM.