All good questions.
1. How many on average will a hen hatch at one time?
Hens and eggs come in different sizes. A very small bantam may have trouble covering 4 regular sized eggs. A large fowl hen can cover a lot more. Different hens have different shapes. I normally give a hen 12 eggs of the size she lays but I’ve had a few where 10 was a full load for her. I’ve seen a hen hide a nest and hatch 18 chicks. I never did fine her nest so I don’t know how many eggs she had.
2. If I let her hatch her chicks would it be ok to let her just hatch 2-3?
Not all eggs that are set always hatch. You never know how many chicks you will get or the sex of those chicks. That’s one complication. The hen will be quite happy whether she has one chick or 18. You need a plan for all the chicks she might hatch (male and female), it’s always possible every egg will hatch, so never set more eggs than you want chicks. Still, chickens are social animals and after the hen weans her chick or chicks, the chicks will sort of form their own sub-flock until they mature enough to force their way into the pecking order, so I’d like to have at least two chicks.
3. Should you keep the chicks and hen away from the rooster and other hens?
People do it all sorts of ways. Some isolate, some don’t. I let my hens raise the chicks with the flock. How much space you have makes a difference but if space is too tight for a hen to raise the chicks with the flock you are probably going to have an awful time integrating them later.
You are dealing with living animals so no one can give you any guarantees. We can tell you what our experience is but you are unique with unique animals and a unique set-up plus your management techniques may be different. If you introduce the chicks to the rooster while they are still chicks he is probably going to see them as his offspring. It is possible he will help Mama raise them but at the least I’ve never had a mature rooster ever threaten a young chick.
Most of the time my other hens don’t bother the chicks unless the chicks invade their personal space. Then they tend to peck them to teach them it is bad manners to bother their betters. The chick runs back to Mama and everything is fine. Some broody hens tend to keep their chicks really close, some let them roam further. Some broody hens attack any chicken that gets close to their chicks (except the dominant rooster usually) some are more laid back about it, but I’ve never had a broody hen that failed to protect a threatened chick. Some people say they have and I believe them. You are dealing with living animals, no guarantees of anything.
4. If I decide to incubate some eggs that hens won't go Broody, how to do tell if their fertil? I know you need a rooster but how do you make sure the egg is fertil before you incubate?
There is no practical way for us to ensure an egg is fertile without opening it, then you can’t hatch it. But you can open an egg and look for the bull’s eye. If the eggs you open have the bull’s eye, then the eggs you don’t open should too. This thread might help.
Fertile Egg Photos