Do you have a sump, or are you reffering to recirc in the tank with powerheads?
If you are reffering to powerheads to provide motion in the tank, then 20x (2000gph) would be good. Of course, placement of the pumps can be efficient or inefficient, erego, it may require more or less.
The flow from a sump to the tank would have to be limited by the capacity of your overflow.
Is that what you're looking for?
Gator