Wow, that's a LOT of roosters. I've heard guidelines anywhere from 6-12 hens per rooster.
Our rooster count fluctuates occasionally around here, but we've had anywhere from 1 to 3 roosters in a flock of around 25. Even with just the one rooster, our eggs have always been fertile and the flock well organized. Right now, we have 3 roosters in a flock of 24, and thus far things are going okay, but we will definitely keep an eye on things, and downsize roosters if trouble starts.
Our flock lives right in our backyard on our small farm, so for us it's very important that we have nice roos and that they all get along-- cockfights and roosters attacking us would seriously impair our quality of life!
If it were me, with 12 hens, I would only have one rooster. MAYBE two, at the absolute max. Eleven? I'd have Sunday dinner for weeks. . .