I tend to cap at 2 or 3, and I do try to stick to those 2 or 3 until I'm done with them.

I'm currently on three: Fable 2, Final Fantasy VI (Let's Play) and Final Fantasy Tactics: War of the Lions.

I have to say, it is unusual for me to play two games of the same series; I tend to stick to one Final Fantasy if I can, but VI and Tactics are very different, so it is okay.