I only play one table if it's a "zoom" table because I can make it go as fast as I want. I also want to make notes about the other players as I go.
If I'm playing a normal ring game, I only do one or two because I want to pay attention to the other players and try to get a bead on them.
If I'm playing freerolls or cheap MTTS I'll play up to four at once, because why not? I'm also limited by the screen size of my laptop, I won't play on a tiny window. Hurts my old eyes.