Well basically, it depends of what you play (SNG, cash, etc), you skill-level, experience multitabling etc...
Also with personal preferences (I like playing 4-6 tables NL SH so that I can focus on each table, but there are people playing much more than that).
You should try adding tables one by one, and see what you're comfortable with. Of course, play a number of hands (5-10 thousand at least) with 2 tables before adding another one. And the same with 3 etc.
When moving up the limits, I prefer to drop to 2 tables for a while, especially if taking a shot (not having the full bankroll needed for that limit), and add it up again.