They tend to grow at different rates, so it's hard to say for sure. I'd order at least pack of fuzzies and a pack of pups. You could throw in some weaned and/or smalls too if you want to be super prepared and take advantage of their flat rate shipping special right now. If he gains weight quickly, most growing balls will take more than one rat per feeding (not at the same time - completely swallow one, offer another) so there's no reason to worry about having small sized rats left over as he grows.