Various sites recommend having about 10 hens per rooster so that the hens don't get over-mated. Chicken mating is pretty rough on the hens, and the degree to which hens suffer seems to vary by individual rooster. I have had roosters who never pulled any feathers out of his hens' backs, and I have had other roosters who plucked a few bald. I run my chickens in tractors, and my tractors aren't big enough for 11 birds--so my hen to rooster ratio is always lower than 10. I have seen others with substantially larger flocks, and they still had one or two birds with bare backs--so even with a large harem, roosters seem to have favorites that get mauled.
There's not much to do to change rooster behavior (as far as I know), but there are some things that can be done to protect the hens. One of the most obvious is to give them a saddle or flak jacket. It straps on their wings and protects their backs--some have shoulder pads, as well.
I've tried a couple of different manufacturers; there are many that are available on Amazon and elsewhere. The price varies widely. Until this year, I have never needed them, but I now have one rooster (formerly two) who is drawing blood when he mates with his go-to hens (others in the flock are fine). These seem to be working well so far.
The hens adjusted to them quickly, and it made an impact on their behavior within 24 hours. The hen pictured above with the shoulder pads had been nicked on her side by the rooster's claws and she had started to avoid coming out of the tractor. A day after getting the saddle, she was back out foraging with the rest of them. In this tractor, two of the five hens are currently saddled.
Hopefully the hens can grow back their feathers and avoid any further injury.
