Trick question: The answer is none.
The American Mustang is not a wild animal. All of the "wild" horses in America are, in fact, feral horses. (The Chincoteague and Assateague ponies are even less wild, they're a managed herd).
Although horses evolved in north America, they became extinct here after crossing the land bridge into Asia, where they were domesticated. Every horse in North and South America is descended from animals brought over by European settlers. Some of them got free, and because the "Mustangs" were useful as remounts, they were allowed to live.
They are now semi-managed herds, with numbers controlled and select animals brought in and trained as working animals. They are not, however, "wild."
Part of a Mustang breeding herd in Arizona. Image source John Harwood via Wikimedia Commons.