I try not to GM more than three games at a time, while joining no more than an additional two games that I don't have to GM. I used to limit myself a little more back when I started RP'ing in high school and just stick to GM'ing one with a cast of 12-15 active players. Most I've been in at once including 1x1's would be around five or so.
~KL~
~KL~