While by definition there will be an average number of follicular units/grafts used. As a result the amount of donor. It can only really be calculated if parameters are equal. For instance, the donor area starting hair density and surface area of the donor. But, not mentioning graft numbers in the question is correct. As the graft numbers are a conclusion of several important factors. The surface area needed to be treated and the natural hair characteristics. Hair characteristics have a huge impact on the distribution of the hair units and ultimately the numbers used.
Coarse, wavy, low contrast with the skin complexion. Fine, straight, and high contrast with the skin complexion. These opposing hair characteristics could potentially double how much donor is required. Included in this is the average number of hairs per follicular unit. Single hair units up to four hair units. This can be influenced by the area the procedure is being performed. For instance, a hairline is going to require a higher percentage of single hair units. Making up the immediate few mm´s of the hairline. This % will drop the larger area is being treated.
The density placed can vary greatly. A minimum density is required to block the scalp and create the important illusion of hair density. Compared to the surrounding hair. Increasing the hair density will increase the donor area/hair used. Some prefer to place a higher hair density. Also, covering any miniaturised hair along the perimeter of the treated area. But also going deeper into the existing hair. This again will increase the use of the donor. This can be argued to be unnecessary as future hair loss could not be predicted to that extent. Using maybe 50% more than would normally be required.
There are then planning options to take into consideration. Some of which are decided simply by the Doctor's ethos on hair restoration. How do they personally perceive a hairline should be designed. Some lean toward a more aggressive design. For instance, closing off the temple points. Which can be very striking, given the correct facial bone structure. Alternatively, a more long-term approach may be to consider the individual´s age and potential for further hair loss. Including simply the ethos that a hairline does not need to be aggressive to frame the face well and look perfectly natural.
When choosing your Doctor these should all be important considerations. As many Clinics can perform a perfectly natural hair transplant. Aspects such as planning, graft distribution, and donor hair management start to become real areas to be considered. These are all questions a Doctor and clinic should be able to answer and justify for you when researching.