I'm assuming you want her to hatch eggs and you have a cockerel or rooster, so the eggs she got are fertile. How old is this chicken and is she a proven broody?
First, since you aren't sure how long the eggs have been there, I'd candle tonight and see how far along they are. Keep at most 9 or 10. If they look like they're at very different stages of development, then you're going to have problems with a staggered hatch. You have a couple options then.
Either throw out the ones that are at very early stages of incubation and keep the ones that are farther along, or let her keep them all and once she has left the nest with the chicks that hatched first put them under another broody or in an incubator. The problem with staggered hatches is that after the first chick hatches the hen will only stay in the nest at most a day or two before giving up on the other eggs, whether they're good or not.
Second, mark all the eggs you keep under her. If she's really broody then she won't be laying. But other hens will still want to lay there, so you can either close off her nest and let her out at least once a day to drink, eat, poop and maybe take a dust bath or risk letting other hens lay in there and crushing the broody's eggs. I don't advise moving broodies as too often they'll just abandon the nest.
Third, broodies are usually good at what they do. Try not to worry too much. It's really cool to watch a mama hen show her chicks how to scratch and take a dust bath! A good broody really would give her life for her chicks.
Hope all goes well!
