People always like to use specific numbers for this, but it really depends on what the rooster is like. I have 5 roosters with around two dozen hens right now: One is the lead rooster: he's not overly macho, and he's not aggressive, but he protects the hens. The others are super, super nice roosters, the nicest I have ever had, and they totally just leave the hens alone, maybe just chasing after them once in a while. If you have a rooster who actively chases the hens, and attacks other roosters, you'll probably need more hens for him, but if you have more of a docile roosters, he may be fine with just one hen. It totally depends. The more aggressive rooster may be fine with one hen, if he doesn't beat her up, and the docile rooster would probably do fine with a dozen hens, and maybe some other roosters.
Throughout the summer, I have had a few other roosters, and any rooster that was being too rough with the hens would get penned up in the orchard until I could sell him to a good home, or he would become better with the hens, which often happens over time. If you do have any roosters in a 'bachelor pad' though, they need at least one other chicken with them, and they should be socialized with the other chickens once in a while.