If I remember correctly the rule of thumb says 8 to 10. My roo has 13 girls and I still have a high fertility rate.
It depends on the individual rooster. Some just have more vitality than others. Older roosters seem to slow down. The bantams are normally more active than the full sized fowl. The rules of thumb I have seen are 10 to 12 hens for a full sized rooster and 12 to 15 for a bantam. I'd expect a normal active rooster to handle more than this, but these numbers are supposed to give you a lot of assurance they are all fertile. Gritsar has Brahmas, a large full-sized breed. By the guidelines, 13 should be too many since the Brahmas are so big, but it is obviously not. Of course, her rooster Thor is still kind of young. Growing up, we had a mixed flock of full sized chickens (they were mutts but from RIR, Dominique, Leghorn, and such stock) and often had a ratio of around 20 hens to one rooster. Most of the eggs we set hatched. I don't know how many of the failures were due to fertility or something else.
