How can I get more cows to spawn?

I don't know how long they take to spawn, but if you need more cows, have you considered breeding them?

You'll need to get two cows - they'll follow you if you're holding wheat in your hand, so you can get them in a fenced in area (that just makes things easier). When you have your cows, with the wheat still in you hand, hold it on one cow until hearts appear. Do the same to the second cow and a baby will appear. Using this process, you can get as many cows as you need and never run out of meat and leather!