I'm guessing somewhere in the neighborhood of 20 cat balls.
Forest and I actually have a little ritual concerning cat balls under the couch. He just LOVES to knock them under there. After a while, though, all his favorite ones get stuck under there and he can't get at them. So he'll sit by the couch, stare at it, and meow piteously. I'll eventually give in and go get the Swiffer. As soon as he sees it, he gets all excited...swished his tail around and rubs all over my legs. I'll take the Swiffer and rescue the trapped pouffy balls. About 10 seconds later he starts knocking them under there again. We call the process a "Pouffy Ball Safari", and it happens about weekly at our house.