That ratio can vary widely, depending on the personality of your rooster(s) and how much experience you have in interpreting and managing their behavior. Some pairs or trios of roosters can get along in a flock of hens, some can't. The more hens, the happier the rooster. The more roosters, the UNhappier everyone - including humans.
Not at all. I can't think of any good reason to have more than one rooster with up to 15-16 hens, and only a high predator load or for breeding purposes would you NEED more than one rooster in a flock of 20+.
Currently, I have four - with 37 hens. It's too low a ratio for my flock, and one of them will have to go unfortunately. Goldie and Roofuss have made a formidable pair and work well together to manage the flock. Blackbeard, the youngest, seems to be finding his place and has been observed working WITH the other two while occasionally being chased away but not flogged. I'm still deciding about him. Colonel Sanders, however, has been beaten so many times he hides inside or underneath the coop (a camper on blocks). He doesn't mate very often, is afraid of his own shadow, and ranges out all alone with they're out of the run. Poor guy, but he's got a date with the chopping block.
You only need a rooster to warn of predators when they free-range, or for breeding purposes. Hens will lay eggs whether there's a rooster or not.
You can integrate him at any time after about 8 weeks of age. But if he's the ONLY chick to be integrated, I'd wait until he's a little older. Either way, follow good integration practices and don't just turn him loose.