The adult human scalp contains 100,000 follicular units on an average. Baldness manifests when around 50 % follicles in the given area are lost. The density of a non-bald scalp is approximately 80-100 follicular units/cm2. This density varies from person to person.
In a single session, hair transplant can cover up the bald area, however it cannot give natural hair density. Doctors transplant 25-35 follicular units (FU)/cm2, which is much less than the normal density, however with skillful planting and careful planning the bald area can be covered up well. To attain normal density, a second session of 25-35 FU/cm2 is required. Up to three sessions may be required to attain desired density.
In the front hairline, solitary FUs are grafted to obtain a natural look. In this area, 35-45 FUs can be planted per cm2. However, in the crown and vertex area, the double and triple FUs can be transplanted to obtain satisfactory results
In an over enthusiastic effort if FUs in excess of 40 to 50 are planted per square cm, then the chances of hair growth are negatively affected due competition between the grafts for blood supply. Those follicles unable to obtain adequate blood supply will die with associated skin necrosis. That leads to permanent loss of the follicle with scarring.
Doctors are trying to push the limits of hair density as much as possible while remaining within limits of the vascular supply.