I used to give myself a limit of only roleplaying in a maximum of three at a time. Any more and I would get too busy. That being said, I pretty much always break that rule. I think I am in about six roleplays at the moment, but half are dead or dying, and the rest are pretty easy in terms of what I have to do for writing.
When it comes to being a GM, I never really wan't to GM more than one at a time. Keeping track of so many elements at once is not really easy, and I'm envious of those who can juggle that much.