The size of Starfleet could vary somewhat. With a large number of inactive reserve ships to call upon if needed. We know there is a reserve activation clause for personnel so they can recall people back to man the ships.
Well it is stated in undiscoverd country that ships and stations can be moth balled.
So the borders could be like I said above have a very low denisty of ships but in extrem circumstances mothballed ships can be reactivated and brought in to increase the number of ships. So that wing of 5 ships could go to 10.