I've got 15 with a Black Australorp right now. She is doing well with them. I have seen as many as 18 with a barnyard mutt hen. I don't know how big your bantam is. Not sure how cold at night it gets in Southern Utah this time of year, but as long as the hen can cover them all when they are about 2 weeks old, you should be fine.
I don't know what your set-up is. I've had success giving chicks to a broody setting on plastic eggs at 4:30 in the morning. I've also had success removing a broody from her nest with a few chicks, putting those few chicks in a box with other new chicks I want to give her, then letting them all out at the same time in their new enclosure. They are living animals and about anything can happen, but most broodies will accept the new chicks without too much trouble if they are about the same age as hers. Just try to mix them in with hers some way so they don't especially stand out.
Good luck!
