Most of the time if you need to exceed the required amount of "pet" dogs allowed, then you just need to be approved for a kennel license. Then you can keep a larger number of dogs with out being fined for non compliance.
It depends on where you live and the laws there. If you are planning on fostering I am assuming you are going to be working with a shelter or rescue, right? They should be able to tell you what paperwork you need to do.