Generally it’s 1 rooster for every 8-12 hens. It depends on the rooster. You might want to have 2 boys to have a higher fertility rate, as long as they don’t start over mating the hens.
Is it advisable? For most people, no. Especially beginners, which I don’t think you are. I have had 9 cockerels, and I’ve had to butcher all but 3 of them, since they were aggressive. I just butchered my last two mean ones earlier. One of the three is a silkie cockerel, and silkies are known for...