[BrandiFan]

I am fairly inexperienced in cash games, but I think there are alot more tough decisions in the cash games since you're always deepstacked (and shorthanded in your case). That in mind, I'd say start playing the same number of tables that you were playing in cash games until you're comfortable with the different style and then add a few tables.
I think if you can play 4 6 max cash tables you should be fine with 8 or more mtts.

edit to add: fwiw I usually play 8 mtts at once.