I’m not a believer in magic numbers, mainly because I’ve seen enough to know that magic numbers don’t work with chickens in much of anything. There are way too many variables with living animals and the ways we keep them for the same magic number to work for everyone in all conditions.
I try to always recommend that you keep as few roosters as you can and meet your goals. For some that is zero and for some it is a lot more. It’s not that more roosters guarantees you will have problems, just that problems are more likely.
The only reason you need a rooster is so the eggs will be fertile. Everything else is just personal preference. Lots of people would not think of having a flock without a rooster. Lots of people are extremely happy to not have a rooster in their flock.
One huge component of this is space. If they have a tremendous amount of space, it is pretty likely the roosters will split the hens up into separate flocks and will be able to peacefully co-exist. By a year old they have made it past the adolescent years so most of the drama is probably in their past but especially if space is tight you could still have plenty of drama. Most people on this forum don’t have the room to support multiple roosters in the long haul. If you have enough room so they can get out of sight of each other, the roosters will probably set up their own territory and get along. The smaller your space, the higher the risk.
Breeders often keep one or two hens with a rooster throughout the breeding season without a hen becoming barebacked. Some people have flocks with one rooster, over 20 hens, and still have some barebacked hens. There are different possible causes for a hen to become barebacked, not all are the fault of the rooster.
If roosters are going to fight, they will fight whether there is one hen or a hundred in the flock. The hen-rooster ratio really doesn’t matter.
I have no idea what your goals are, your management techniques, or how much room they have. For the vast majority of people on this forum I’d strongly suggest no more than one rooster with five hens, but if your conditions are right you may be OK with 3 and 5.