Just as important as how many leaves, you want each leaf to slide easily on the leaves next to it. You can improve the ride of a car a lot by simply smoothing the surfaces of each leaf with a body grinder and tapering each end so it slides easily over the next leaf.
It also helps a lot to put teflon or plastic strips between each leaf, or even to put window screen with grease all over it between each leaf and then taping the entire spring with electrical tape to keep the grease in.
You basically do not want any friction between the leaves.
Don