As many as you are comfortable with. Also depends on format: SNG easier to multi, MTTs harder and HU very hard, due to volume of hands. Alot of HS HU regs only single table. Personally I started with 1, then 2…3 and now 7. Although this is nothing compared to alot of 180 man regs. I know 2 who 40+ table every time I check them out. 

I would imagine most MTT players stick to 15 or less. When I last checked out the stats a year ago, the average seemed to be 10, although the general consensus  seemed to be that most folk felt playing 4-6 was ideal for their A-game. 

I have absolutely no idea about cash. I expect, as they seem to feel, that cash game specialists are the greater artists  when compared to MTT-ers, (which does not equate to greater skilled btw), but cash poker bores the living be-jeezers out of me. The excitement of tourneys is what keeps me coming back. 

In my opinion, the tournament player equates to Indiana Jones, and the cash game player is Steve Davis (the snooker player…Google a photo of him and you will immediately understand).