Refer to this post dated 10/09/18 at The Daily Pitchfork for livestock grazing on lands managed by the BLM. Grazing receipts from 2002 to 2018 were used to estimate the number of livestock on western rangelands vis-à-vis wild horses and burros.
The report did not indicate how many livestock were on lands set aside for WHB so comparisons of cattle and sheep to horses and burros may not be valid because they don’t have the same land-basis.
If all of the animals were found inside the 27 million acres managed for WHB, then yes, you could say that cattle and sheep vastly outnumber horses and burros.
Western Horse Watchers Association 