Well, initially I would say it depends on the Role-Play.
However, as a general rule of thumb, in IRL games (D&D, Call of Cthulhu, etc.) you don't want more than 8 players at a time, not including the Dungeon Master. So I'd say that translates pretty decently to the role-play: no more than 9 people. It's diverse enough that you don't have to necessarily interact with someone, but you can also keep track.
Other things factor into this though, like:
-Number of Posts
-Quality of Posts (estimated) from a given Person
-What the RP is about
-If larger (or smaller) numbers make sense
-Attention Span of given RPers
-Etc.
Still, this is an interesting question to discuss.