How common is it for your uterus to be punctured during a dnc?

Not overly common. The uterus can readily be punctured during a D&C or hysteroscopy, for ex. Even an endometrial biopsy, done in the office, has uterine puncture as a risk. There is a higher risk if the uterine position is tipped back or immediately after pregnancy biopsy. Typically, the hole just closes - without treatment.