I play 4 cash tables. I dont really recommend playing any more.
Having more than one laptop or screen helps. In party poker
you can resize to 4 in a screen but only in holdem, so I have to play 2 per screen (Omaha).
If you play holdem cash I would probably say 3 tables. The reason being that Omaha really doesnt have any equivalents to low or middle pairs or KJ/KT/QJ etc, so 4 is OK to follow.
If you play MTTs I think 2 or 3 is the maximum really, because of tables changes etc and the need for closer attention.
The main reason I think it is necessary to multi-table is funnily enough not so much to do with money, it is more to do with the alleviation of boredom. Anyone who plays a large volume of cash hands knows one thing, it is very very tiresom to do it correctly. If you dont multi-table there is too much of a danger of you going "thrill-seeking".
Anyway, hope this helps