Similar to others I'm seeing in this thread, usually 2-3 single player games at any time otherwise I just lose track and find i don't get as invested into any of them. I don't count multiplayer games because I generally use them as a social experience or just to fill short windows of time.
I don't have any particular rule as to what the few games I'm playing are, but usually they'll be quite varied to break things up a bit, so it could be one RPG, then a simpler shooter or arcade style game for when I'm not feeling like playing the RPG. I'm also trying to break out of the habit of feeling i have to complete a game, if I'm not drawn back to a game I shouldn't force myself to play it for completionism's sake.