I have about as many hens as you do, it varies from year to year, but I like to go into winter with a dozen give or take. I have been quite happy with one roo. I do not hatch out year round, but I do need the protection an adult roo does bring.
I free range in the wilds of SD prairie and every predator is there. I have fort knox, coup/run.... but I like to let them out, and therefore lose some.
With a good adult roo, I have much less day time losses. Not all roo's are good. Right now I am without a roo, and when you only have one rooster, that is a chance you take, you can spend part of the time without a rooster if something happens to your main boy.
However, if you have just a backyard flock, I think one is enough.
Mrs K