Naturally, the more table you play the smaller your ROI gets. How small depends on 1. your skill level, 2. your ability to multitask, and 3. if you have any tracker aids (PT w/HUD for example). If your ROI is around 15% with one table (say at $5 SNGS), and only goes down 1 or 2 percent with a second table then it is still very profitable to do so.
When I have the time I feel comfortable multitabling 4 sngs at once, usually $5 or $10. I stagger the starts so I don't end up in high blind or heads up play at several tables, as it can be very fast and frustrating. Or start a turbo and a regular 9 table at the same time, as the turbo will end much quicker.
In tourneys and CC events I usually only play the one table, as it gives me time to take notes and observe other players actions (like pigpen already stated). You lose that edge when multitabling.