You count the number of hairs in a given field and divide it by the number of follicular units (the units are the bunch of hairs and sometime the bunch has a only single hair). That will give you the average number of hairs per graft.
Left Box has 33 hairs and 16 hair groups – 2.1 hairs per follicular unit
Right box has 38 hairs/18 groups = 2.1 hairs per follicular unit
These are two different views from the same patient so we can assume that this patient has 2.1 hairs per follicular unit across his entire donor area.
The total number of follicular units on any mans head is 50,000 (average size) and 20% of that number can be safely transplanted, so this man
(assuming that he has an average hair thickness) has a total permanent zone of 10,000 follicular units of which 60% (or 6,000 grafts) can be safely moved. This number rises in coarser hair and drops in finer haired people. All too often, men and doctors push up the number of grafts that can be moved at the risk of depleting the donor area, especially in FUE cases.