First of all it's not really an "exemption", even though everyone refers to it as such. Technically, it is an open space valuation. In other words they don't exempt you from taxes that are due, they actually value the land lower. There is no provision in the code for the number of cows per acres. What it says is:
"Land that is currently and principally devoted to agricultural use to the degree of intensity typical for the area and has been used for agriculture or timber for at least five of the preceding seven years."
The key term is "typical for the area..." And your county extension agent is the arbitor of this. Around my area one cow on 20 acres isn't going to pass the smell test, but out in West Texas it probably would.
Also keep in mind that you don't have to have any cows on it. For instance if you or someone else is cutting hay off of it for agricultural use then you qualify. If you have any doubts contact your county extension agent.
CJ