One roo can easily service 20 - 30 hens, even more. However, many folks keep a roo and a couple hens as a 'trio' in their breeding programs. I have a big main flock but we also have a polish roo and his harem of three ladies. So we have a quatro (?) which I am hoping to rehome very soon. LOL
If you don't keep an eye on his spurs and try to keep them trimmed down he can seriously harm your hens when he mounts them. That is one of the reasons some people have chicken saddles on their hens. The feet and spurs when they mount not only will pull out her feathers but long sharp spurs will cut her to ribbons. It's not that he won't be able to mate, you will have to move your hens out when he damages them and he won't have anything in there to mate with.
Heed his advice but for a different reason.