For 30 hens 3 roosters would work. 1 rooster for every 8-10 hens is considered "normal" - however, this is not a "written in stone" number ... it can vary with breed of chickens, space for chickens and how the roosters get along.
I would get two or three roosters for 30 hens. I've read that a rooster can effectively cover 1-16 hens in his prime. When he reaches the age of three, he is already past his prime. So, I guess it depends on how old your roosters are going to be. If you get young ones, you might be able to get away with two for now and add another one or two as they age.