Without reading anyone else's replies to this thread (but already knowing what they will tell you).....ONE. Move up from there when you feel ready, when you're beating the game on a regular basis.

At some point you may get bored with one and try two. Two tables will seem CRAZY sometimes, I know that it did for me when I first started out. Slowly, but surely I was able to move up to 4, 6, and ultimately 8 tables. It takes practice to be able to move up to those levels...