Hm. I wanna say 3-7.
2 people is probably fine, but because it has a GM role I'd want there to be at least two players to be able to bounce off each other.
8+ would also work but at that point the mechanics might be a little involved for the pacing you'd want. If I were gonna run a group that big I'd probably try a game called Freeform Universal for a similar but simpler experience.