This question comes up all the time among beginning cattle people even experienced cattle people have to make an educated guess when first stocking a place. A couple of ideas for finding the your best stocking rate come to mind. First contact your local Gov. Farm service office/soil office/ usda office. Second if you can find the previous renter/owner he might give you some idea. Third ask a neighbor with similiar looking grass how many he stocks and if he fertilizes. After you have gathered this information you can make a educated guess that should work ,good lord willing.
Remeber that many factors go into stocking rate such as weather , type cattle, supplemental feeding, fertilization, cross fencing, type grass, etc .