Nobody really knows for sure. This is because horses are not counted in all the areas where they roam.
But, the BLM does publish estimates of the number of wild horses and burros that roam on public lands that it manages. As of February of 2007, the BLM estimated that there were 28,500 wild horses and burros on lands that they managed. It is likely that there are fewer than 25,000 horses and fewer than 5000 burros left on 34 million acres of public land managed by the BLM. That’s one horse for every 1360 acres!