As Emma said, that depends on the birds. My English Orpingtons could have three males on 12 hens with no issues provided they all get along. Generally speaking Welsummers, Rhode Island Reds, and Easter Eggers are all quite....virile birds. I would advise only one male of those breeds on 12 hens. If you had something more docile like a Cochin you might get away with two.Can a flock of 12 hens manage two roos?