I agree, one standard rooster should be a good number for 12 hens. Right now, we have almost 50 chickens, about exactly half of them are roosters. But, we have separate pens so that we don't have too many roosters to hens. We have one pen, with our pullets in with one big rooster, and 5 small bantam roosters. The bantam roosters aren't a problem, because they aren't big enough to mate with the pullets or stress them out, and our big rooster isn't bothered by them because he knows he is bigger.
As was said above, it depends on why you want the rooster. For breeding? To protect the hens? We have a cream legbar in with our 12 older pullets, it is a more docile breed, and a good one for breeding blue egg layers and olive eggers. He also does a great job protecting the hens and showing them snacks. Technically, because of our bantams, we have 5 roosters in with 12 pullets, which a lot of people wouldn't recommend. But the bantams get along find with everyone, and they are super friendly with us and the other chickens. Our bantams include 2 porcelain d'uccles, 1 buff cochin bantam, and one old english silver duckwing game bantam. So if you want one standard rooster, and then add some bantams in for ornamental or cuteness purposes, I wouldn't recommend against that
We then have a rooster flock where there aren't any hens with for the other roosters that we want to keep. We have some younger chicks, and when they grow up, we will make another "hen pen" with one rooster in for breeding and protection. The other roosters will join our rooster flock, out of sight from the hens. If we want to breed the hens with another rooster we have, we will but them in a breeding pen for a couple hours, let them do their thing, and return them to their flocks. This way no one will forget them, and it won't mess with the pecking order.
Also though, you don't necessarily
need a rooster, if you don't want to breed your hens. However, note that if you don't have a rooster, you will likely get a dominant hen that takes the place of a rooster. The hen may crow, be aggressive, or even mount the younger hens like a rooster would. This is why I feel like things are more balanced when there is a rooster in the flock, but some people choose not to.