Once you reach the desired number of critters, just stop feeding them. When you cull them, feed the remaining and build your population again. There is a limit per area. I usually pen off an area that's about 3x5 squares and I tend to end up with about a max of 10 boars, no matter how much I feed them after that point. Wolves are far more prolific and will fill a small area very quickly with many more.