I suppose you could do that with just 1 acre but 2 would be a little better for when/if you add more animals.

http://ohioline.osu.edu/gsg/gsg_2.html

Check this. It breaks it down for you.

it would depend on the type of land and grass growing on it. If it is hilly or rocky land than u would need more than if it was good flat grass land. Also don’t forget about water.