Others have been doing a good job but I'll chime in also.
Should I collect all the eggs from all the chickens and put them under the broody? Some will be fertile and others won't right?
If you have a rooster for every 10 to 12 hens, all the eggs should be fertile.
How many eggs should I put under the broody?
As mentioned, it depends on the breed and size of the broody. Some bantams can only handle 3. Some full sized hens can easily handle a dozen. She needs to be able to cover them all to keep them warm. For your first time, I'd go with a few less than the maximum.
Should I put the broody in a separate area so she is by herself with the eggs?
I'll copy something I recently wrote in another post that expresses the options as I see them.
I don't know your goals for chickens or your set-up. I'll try to give you some basic information so you can decide what to do that is best for you. What you do might be totally different than what I or someone else would do.
I see two options for you, either move her to an isolated pen where the other hens cannot get to her nest to lay or leave her where she is.
If you leave her where she is, you need to decide what eggs you want her to hatch and mark them clearly. A black magic marker works great. After you have collected and marked all the eggs you want her to hatch, you put them under her at the same time and remove any eggs that are already there. If you wish, you can put fake eggs, like golf balls, ping pong balls, plastic eggs, whatever, in the nest for her to brood while you are collecting the eggs you want. Then, at least once a day and I consider the late afternoon the best time, you need to check under the hen and remove any eggs that are not marked. Chicken eggs take about 21 days to hatch, some a bit less, some a bit more. When a chick hatches, it has absorbed the yolk and can go about three days without food or water. This allows the early chicks to stay on the nest until the late chicks hatch. If all the eggs are not started at the same time, the hen has to decide if she will let the chicks that have hatched die of thirst and starvation or leave the nest before all the chicks have hatched. She'll take the living chicks and leave the unhatched eggs.
The other hens will continue to lay eggs in the nest with the broody hen. You have to remove these daily. If you don't, bad things happen. The other hens can lay so many eggs that the broody hen cannot cover them all. When that happens, an egg that was developing is not covered, it cools off, and the chick inside dies. Then, that egg gets moved back under the hen, another developing egg gets moved out, it cools and dies. You will not get a good hatch when the hen has too many eggs. You also get the staggered hatch, which gets real complicated and often does not end well.
If you mark the eggs and remove the new ones daily, you will normally get a good hatch. However, some things can go wrong. Other hens can break eggs if they are crowding onto the nest to lay eggs. It does not happen a lot, but it does happen. The broody will normally leave the nest once a day to eat, drink, and go poop. If she comes back and another hen is on the nest laying, she may get confused and go sit on a different nest. Her eggs then cool off and the chicks inside die. Again, it does not happen a lot, but it does happen.
The other option is to move the broody to an isolated pen. This needs to be an area that is big enough for you to give her food and water and she can leave the nest to poop but the other hens cannot get to her nest to lay. Obviously, all the eggs need to be started at the same time for the same reasons as stated above. This is the solution I use.
There are some drawbacks to this method.
The hen may break from being broody when you move her. I'd suggest putting fake eggs in the nest for a couple of days before you give her the eggs you want her to hatch to assure she will stay broody.
You have to alter your routine to separately feed and water her daily. For most people this is not a big deal, but I don't know your normal practices.
A broody holds her poop all day and when it comes, it is big, messy and stinky. You'll need to keep her pen clean. This can discourage some people.
The pen obviously needs to be predator proof. Not always as easy as it sounds, especially if you do not have room in the coop.
I like to keep the pen in the coop or run with the other chickens so she stays a part of the flock. Otherwise, when she rejoins the flock, you wind up having to reintegrate a new hen. Often this is not a big problem if you put her with the flock with her chicks. Mama has such a bad attitude and will defend her chicks vigorously so the other chickens tend to leave her alone after a skirmish or two, but it can be a problem.
As I said, these are the basics as I see them. Others may have different opinions and experiences which I hope they share so you have a better chance of making an informed decision. There are strategies to overcome some of the problems I've mentioned but I'm trying to keep this simple.
