Doug Polk recommends maxing out at 5 or 6 tables.
BlackRain makes note of how winrates drops as the number of tables is increased. As you can see, some people play over 20 tables at a time.
BlackRain's take on win rates
That said, you should be able to easily play 2 or 3 tables with no loss in ability. I would recommend getting your game as solid as possible and sticking to one table if that is what you feel comfortable with. Once you get better, one table will become extremely boring and you will open up a second. Then that will become boring too. Lather, rinse, repeat until you find what works for you.
And no, I don't think there are any online pros playing just one table.