Get the fertile eggs under her as soon as possible. Switch them out at night with no lights. She will stay with the eggs until they hatch. Chickens can't count. Buffs are great mothers but my buff has to be taken off the nest to poop. I also have to put food and water right in front of her so she eats. If I don't do that she gets thin and unhealthy. Give her a broody box that is large and comfortable away from the other hens. I also make a 'run' out of chicken wire to keep all the chicks together once they hatch. The 'run' and broody box are in an enclosed area of the barn. The chicken wire won't protect them from predators but it keeps them corralled. Those little fluff balls can hide in the smallest places.
You will know if your buff is pooping or not. A broody poop is huge and foul smelling. For my girl if I noticed that she wasn't doing her business I would gently lift her (making sure eggs aren't between her wings and body) and talk to her and take her outside the broody area. Place her down gently. She might not be able to stand right away because she has been setting for hours. She eventually will get up and do her business. Hopefully she will eat a bit of grass and her food and water. If she doesn't know where her eggs are gently guide her toward them.
While you are waiting for fertile eggs you can treat her just like she has real eggs under her. It will give you a chance to get her used to you picking her up and please make sure she is eating. She will be on the nest longer than usual and sometimes without help hens will starve themselves.
You are in for an incredible treat. Watching a hen take care of her babies is amazing. She will teach them what to eat and will talk to them even when they are still in the shell. When you see that first chick peeking out from under her the feeling is indescribable. When the chicks are about 10 days old they will treat mom like a jungle jim. They will run up and down her back, peck at her eyes and hang on her waddles. Mom lets it go in stride.
Make sure you have a chick waterer, feeder and chick feed. The chick feed is ground a little finer and doesn't have the calcium layer feed has. If they eat layer feed they can develop kidney problems. At 4 or 6 weeks mom will reject them and you can reintroduce mom back to the flock. Read about how to do this. Search here. Reintroduction can be difficult. The babies will need separate quarters until they are old enough to join the flock.
