The vogue in medicine used to be to number the digits 1 to 5, but this gets confusing depending whether the thumb counts as 1 and whether the hand is supinated or pronated. Now its back to being thumb, index, middle, ring, little, with supernumary digits being described in full.
e.g.
The patient has a supernumary digit on the ulnar border of the left hand, adjacent to the little finger.
The patient has a supernumary digit betweeen the index finger and middle finger on the right hand.