Re: Wanted players: silliness.
For an experienced GM, running games is like spinning plates on sticks.
If they're running smoothly with plenty of player interaction, a GM can get away with little more than reading for a while, but when they wobble, they need a little more attention.
The important factor isn't how many games you're running, but how many are wobbling, how severely they're wobbling, and how much attention you can give them - none of which is evident to an outside observer.
As BBR says, the only person who can decide how many is too many, is the GM.
However, we're all tempted to add one more plate, especially when they're all running smoothly and we have time on our hands...