In 10 or 15 years of having a Horseman camera, I've never seen nor heard of a 12 exposure back made by Horseman. If there is such a back, it must be extremely rare.
However, Graflex made a more common 12 exposure back for 4x5 Graflok backs. If you go that route, look for a shell with the chrome pin rollers on each side of the film gate. The pin rollers are about the diameter of the lead in a wood lead pencil. The early ones didn't have them and the early ones don't hold the film as flat. The Graflex backs with lever wind inserts originally came in a shell with pin rollers, but you have to check because over the years, inserts and shells often got switched around. So you could find a lever wind insert in a shell without the pin rollers.