High density- There is about 3 to 4 hairs in a single follicular unit.
Low density- you are only able to see one hair, and occasionally two hair single follicular units.
The goal of a hair transplant is to balance everything while creating a natural look.
When balancing a low density look prior to surgery, you will have an end result of a low density in the recipient area (after the surgery).
When balancing a high density look, you will have an end result of high density. The end result is always dependent on the original density of your natural hair.
It would look unnatural to transplant a thick new hairline and still have a low density of hair throughout the rest of the scalp. An even hair number required will be different .