You don't have a rooster, but are you allowed to?
About half of chicks will be male. But it's not rare to hatch two or three chicks and have them all be one gender--so there's a chance that you will get multiple males and no females. (Equally good chance of getting all females, but that wouldn't be a problem.)
I agree that buying chicks for her to raise might be a good idea, especially if you only want females. Let her sit in a safe place for about three weeks before you try it, because she's not ready to take care of them yet (hormones change during broodiness.)
For how many eggs or chicks: it's not good to raise a baby chick all alone, with no other chickens. If it's being raised by a hen, I wouldn't worry about it, because it clearly has Mom for company. So I would say to not do more eggs or chicks than you can keep, unless you are willing to later butcher some, or have a plan to rehome them.