I don't think you're talking about hatching, just giving her already hatched chicks, but I'll answer that anyway. Hens and eggs come in different sizes. A hen needs to be able to comfortably cover all the eggs. A tiny bantam may struggle to cover 4 large fowl eggs. Who knows how many bantam eggs a large fowl hen can cover.
I once had a hen that could barely dover 10 eggs. I had a hen hide a nest and come off with 18 chicks. I never found that nest so I don't know how many eggs she started with. So no definite number but I normally set 12.
Time of year has a lot to do with how many a hen can raise. It doesn't take them long to grow to a fairly large size. Depending on how cold it gets she might reach a limit here. But I've had hens here raise 15 and 16 chicks in warm weather, plus that one that hatched 18. I think
@centrarchid said he had one raise well over 20. I've seen chicks just a couple of days old sleep next to or on top of Mama instead of under her with the overnight lows in the upper 70's so she doesn't necessarily have to be able to cover all of them. In cold weather you probably want to restrict that total number.
I don't do guineas but I don't see why a hen couldn't raise them. As active as they are she'd probably go bonkers but she'll probably manage.