Just a newbie here (*waves*) coming in late on the conversation.
I've got 1 barnevelder roo in with 12 hens, and when we hatched out our eggs, we ended up with 75% hatching. So if you have a good roo in there, clearly one roo for 12 works beautifully. None of our hens had any sore backs or problems with being torn up, and the whole group seemed happy.
That said, the hens are getting a bit old now, so we're turning the flock over and selling the elderly hens off in groups of four. Chooks are somuch in demand around here that you can get a good price for chooks that are even 3-4 years old and not laying regularly any more (I keep mine for eggs rather than showing, so regular laying is important to me). This is a crazy world - 30 years ago, everyone got rid of their chooks, and now everyone is desperate to get them back!
Cheers,
Leanne in New Zealand.