Commercial operations that furnish hatching eggs use a ratio of one rooster for every ten hens or so full sized hens. For bantams that ratio changes to maybe one rooster for every 12 to 15 hens. This can be in flocks of a few hundred chickens, so a ratio means something with those numbers. Different roosters have different interests and abilities. Averages and ratios don't mean a whole lot when you are talking about one individual. Older roosters are not as driven by their hormones as younger roosters. Many roosters can keep a flock of a lot more hens fertile. It purely depends on the individual.
With your set-up and with 12 hens in each pen, I'd try one rooster with each group. That may not be enough, but it should work out OK. No guarantees though.
Certain breeds with real thick feathers can have problems successfully mating, even if the rooster and hen both try. The feathers can be so thick in the vent area that the rooster has trouble hitting the target. Silkies, Cochin, and Orpington are some breeds I've seen mentioned that sometimes have this problem. It does not happen to each and every chicken in these breeds, but it has been recognized to sometimes happen. Breeds with these thick-feathered breeds often practice vent-trimming. This is where you trim the feathers around the vent to give the rooster a better target. Some people pluck those feathers out during breeding season. If you pluck out the entire feather, including the shaft, the feather will grow back before the next molt. Some people cut the feathers off with scissors and leave the feather shaft in. These will grow back nest time they have a full molt. You can check the fertility of the eggs by looking for the bull's eye. This link shows you how. Sometimes it is hiding on the bottom of the yolk. If it is, you need to turn the egg yolk over gently with a spoon.
Fertile Egg Photos
https://www.backyardchickens.com/forum/viewtopic.php?id=16008&p=6
With your set-up and with 12 hens in each pen, I'd try one rooster with each group. That may not be enough, but it should work out OK. No guarantees though.
Certain breeds with real thick feathers can have problems successfully mating, even if the rooster and hen both try. The feathers can be so thick in the vent area that the rooster has trouble hitting the target. Silkies, Cochin, and Orpington are some breeds I've seen mentioned that sometimes have this problem. It does not happen to each and every chicken in these breeds, but it has been recognized to sometimes happen. Breeds with these thick-feathered breeds often practice vent-trimming. This is where you trim the feathers around the vent to give the rooster a better target. Some people pluck those feathers out during breeding season. If you pluck out the entire feather, including the shaft, the feather will grow back before the next molt. Some people cut the feathers off with scissors and leave the feather shaft in. These will grow back nest time they have a full molt. You can check the fertility of the eggs by looking for the bull's eye. This link shows you how. Sometimes it is hiding on the bottom of the yolk. If it is, you need to turn the egg yolk over gently with a spoon.
Fertile Egg Photos
https://www.backyardchickens.com/forum/viewtopic.php?id=16008&p=6
