"Biopsy" is the singular form, while "biopsies" is the plural form.
There are many times when breast core biopsies are performed. Breast core biopsies are performed when a woman has an abnormal lump in one of her breasts.
Clinical
biopsies
Studies have demonstrated that approximately 80% of all breast biopsies result in a benign pathology report
biopsies
Skin biopsies are usually brief, straightforward procedures performed by a skin specialist (dermatologist) or family physician.
I found that the code 56821 covers both the procedure and the biopsies. This code does not say that there are a limited amount of biopsies that it covers so it has led me to believe that this code does not need a modifier.
Biopsies can be classified as excisional or incisional. In excisional biopsy, the lesion is completely removed; in incisional biopsy, a portion of the lesion is removed.
Pathologist
It is estimated that 75-80% of all breast biopsies resulted in benign (no cancer present) findings.
Skin biopsies also can serve a therapeutic purpose. Many skin abnormalities (lesions) can be removed completely during the biopsy procedure.