We have one roo that has had 11 girls (up to this last week when something whittled our flock down to 9) and one of our girls has feathers missing off her back. She's not bare, but close. She's his favorite and she's the smallest girl, so I think it's how he holds on to her. All the other girls are fine, no missing feathers or anything (well, a few off the back of their heads. :-(The number of roos that can sustain a pecking order depends on a couple of different things. The breed, age, and especially how many hens you have. Personally I would not have more than 1 roo for less than 8 to 10 hens. Young roos, especially when more than one, can over-mate the hens and result in your girls walking around with bare backs (this can happen with even one roo if he doesn't have enough hens). It can help when roos are raised together.