I think limits should be minimal. When I RP on WoW I occasionally dualbox (use more than one program to log in as more than one character) to create interesting conflict, facilitate character development or just give people a villain to beat up and rob. This is a much easier trick to pull on forums, and I don't like having the possibility closed to me.
That said, I'm not particularly adverse to rules that say players should keep one or two main, invested characters, and that they can have characters of lesser importance on the side for the reasons stated above. It's good to keep a few interesting characters with depth and relevance to the plot rather than drowning in showers of throwaways with a name, a face and some intentions.