Diameter of hair or whether hair may be finer or coarser is an individual/different characteristic/data point than donor density (follicular units per cm2).
An average donor density would represent around 70/75 FU per cm2.
The lower the donor density, the less grafts that would be safely available to harvest.
Whether an individual may be a viable candidate for surgery despite a lower than average donor density would depend on many factors such as age, extent of loss of the patient, hair groupings, levels of miniaturisation, hair diameter, objectives, etc.
A patient with less than average density but more favourable hair groupings may be able to achieve a higher “hair count” and so may still be a candidate for surgery.
In summary there is no definitive answer, which again reinforces the importance of a very unique approach for each and every patient.