How common is it for someone to need a breast augmentation or enlargement?

Not need:it's a want. Having breast augmentation is generally not a need but a want or desire and is therefore considered elective surgery in the vast majority of cases and is not considered reconstructive.
Procedure is common. In most cases breast augmentation is an elective surgery, (sometimes there is a need for implants with reconstruction). It is very popular operation with over 300, 000 cases done each year. Best to see a board certified plastic surgeon. Together you will determine if it is right for you.
A personal matter. Breast augmentation surgery is done for cosmetic enhancement in most cases. The exceptions are those who have had mastectomy for breast cancer and the rare individual with significant disparity in the size of the two breasts. Otherwise it is a matter of personal cosmetic feeling and not a medical decision.
#1 cosmetic surgery. Breast augmentation is the most commonly performed cosmetic plastic surgery performed in the usa. So - it's really quite routine and common. Over 300, 000 are performed each year.
Personal choice. Breast augmentation is a personal choice, nobody needs it. However, it is one of the most common procedure that plastic surgeons perform, at over 300, 000/year in the United States.
Refer: Plastic Sureo. Breast augmentation is a personal choice. I recommend you post this question to a plastic surgery physician.