The highest tooth count ever used in an OE Ford application was 21 teeth. More than that, the teeth are very fine, and can wear prematurely. One that the Mustang guys use, and is often sold for too much money by companies like Steeda, is a Jeep gear available from your local Chrysler dealer. It's a 23 tooth, same overall dimensions as a Ford manual trans speedo gear ... part number is J3167393.
cheers,
Ed Nicholson
SCCoO