Interesting that you ask that. I just did that! I have 3 roos to 10 hens (bad ratio, I know! Working on that!) small bunches of different breeds. Now 1 good rooster should be able to keep about 8-10 hen's eggs fertile. If you want eggs from 20 hens to be fertile, get an additional rooster. The roos may need to be separated though. Not always, but sometimes. In short, as long as you're not selling & claiming purebred, there's no problem with that. Mutts are great! Hybrid vigor & all that! So I have some mutts in a brooder right now from hatchery stock parents. None of the chicks are pure, & that's what's so much fun! You don't know what you get, & they will grow & feather out to whatever! The fun is in watching them grow. If any have debilitating deformities, well they don't get to breed. So far my chicks appear perfectly healthy. I have 3 more due Saturday. Yay!