mathematically, it does not matter how many tables you play. if you are adequately bankrolled for a certain level, then playing multiple tables does nothing other than to increase the number of hands you see in a given period of time.

winning players like to increase the number of hands they see in a given period of time. this is because, over the long run, they are winning players. multitabling makes the "long run" come quicker.

the one disclaimer is that most people cannot sustain the same win rate at multiple tables that they can by playing just one at a time. this is expected and ok, as long as the depreciation in your win rate is inversely correlated with the number of tables you add. for example, if you move to 4 tables but your BB/100 drops to 1/4 what it was at one table, then you are making the same amount of money. make sense? so, at some point, there is a diminishing return for most players. see elipsesjeff for more complete analysis of this.