I've always had great fertility with a single roo over a flock of up to 24 hens. If you have a mixed flock, and there is a particular breed you like in that flock, choose a roo to match. Or you might consider choosing a roo who will give you black or red sex links. My last roo (see avatar) was an EE. I chose him for the pea comb and blue egg genes that he would add to my flock. He produced great black sex links with pea or walnut combs that laid blue, green, or aqua eggs. My current roo is a Buck Eye. I chose that breed b/c of the pea comb, and b/c that breed is not common here in Maine. So, I'm thinking that Buck Eye chicks will sell well. He will also produce lovely little black and red sex linked chicks with pea or walnut combs. There is no way I would ever have the space or inclination to have roos to match all the breeds that make up my flock. Instead, I choose hens that will work with the genetics of the roo.