Most momma's are great and will protect their babies from everything. Protecting them other chickens is something they rarely fail at.
However, some aren't so great and/or may have trouble keeping a large bunch of chicks safe but that is usually from other predators (dogs, cats, hawks, etc.)
My broody momma only hatched one chick and she did an excellent job of taking care of him. But, he had her undivided attention. However, I did have to help her protect him from my barn kitties more than once when I first let them out to free range. I just made sure I was always there when they weren't safe in their pen.
