Depends on the roo.
I have an older too that is content with 3 or 4 hens and doesn't over mate them. He's a true gentleman.
I had a young roo. In the crockpot now. That would chase down hens constantly. I don't think 20 hens would have been enough for him.
I have in my breeding pens now 2 different sets of 1 roo to 3 hens. I get fertile eggs and hens still have all their back feathers