There are numerous factors that come into play as to how many RPs one can handle at once. It just depends on the person, their free time, their motivation, etc. One answer is not going to be the same for everyone.

I used to run/play in 15-20+ roleplays back when I was in high school/just graduated from high school. Now in my middle twenties I tend to keep it more at a mellower number than what I used to do. :lol