The 10/1 ratio that "they say" is for optimum egg fertility in large commercial breeder flocks. Your mileage may vary.
Exactly. I was reading about different breeds and one with fertility problems, they suggested for maximum fertility that you keep one rooster per 3 to 4 hens during the mating season.
I had a young rooster in with 18 hens this spring and all the eggs I put in my incubator were fertile and hatched
Several years ago I lost most of my hens in a predator attack and was down to four when I added a rooster to carry over winter and be able to hatch chicks in the spring and it was fine. When I added more in the spring and he then had twelve hens, that worked, too.
It will help if your birds have a big enough area and can free range and have other things to keep them busy. Being confined to a small pen may be a little much.