How many donor hair do we have? What sites (other than head) can donor hair be harvested from? How much hair can be provided?

In general, an average Caucasian male has about 8000 grafts.  This does not mean you can do 8000 FUE grafts at once.  FUE is also drastically different than Strip and there is an issue of donor depletion or thinning where the back of your head starts to look very thin with a short hair cut.  You would start to see thousands of FUE scars.Just remember a 4000 graft FUE scar is like having a collective open wound hole the size of a baseball on the back of the head. There is nothing minimally invasive or scarless about that.

The number of hairs varies in individuals and I would need to see your head to give u a proper answer.

Amount of donor hair for hair transplant varies according to the density of your occiput- the back part of your scalp where hairs are not "programmed to die." Average density in this area is 60-120 follicular units per square centimeter or 100-200 hairs. Scalp hair is best for transplant because of the superior density, consistent angulation, and quality of hairs. Chest hair for example can be much sparser than it looks, with a high transection rate making it tough to use.

The entire head of a Caucasian has 50,000 follicular units. The donor area has at leas 10,000 follicular units, that means by harvesting with fUE half of the donor area, that means that a person would get 5,000 FUE grafts with a depleted, thin looking donor area. If the number was 15,000 rather than 10,000, then a person might get away for 7,600 follicular units for 7,500 FUE grafts.  An Asian is 20% less and an African American is 40% less in terms of maximum yield of FUE grafts. It is not reasonable at the staft of this process to think beyond scalp grafts for FUE for many reasons, poor growth cycles, etc...If you never had a hair transplant, please don't think beyond 7,500 grafts and if you look at my gallery on my website (see web reference below) there are over 1000 patients shown there with before and after pictures and grafts of all numbers to give you the insights you are looking for

One of the problems that arises after FUE is the donor depletion, specially in large sessions.  There are some doctors that recommend BHT or Beard transplant to the head as an option to the donor zone in the back of the head.  However, there is genetic difference between the hair in the back of the head and the hair in other parts of the body.  The hair in the back of the head Has been proven to be resistant to DHT which is the hormone responsible for hair loss .  If a surgeon takes hair from the safe donor zone then the chances are those hair will live long term and will not fall off. However, there are no studies to show the long term life span of the hair that is taken from other parts of the body and so no grantee that the hair will remain. You may go though the process of transplant from other parts and in a few years those hair may fall off and leave you with no hair in those areas. This is a risk that you take with BHT and beard transplant that some doctors overlook to explain to their patients.  In my opinion, see your hair transplant specialist and have them examine the donor safe zone and realistically evaluate how much hair is available for harvesting.  You may be surprised how much hair is left.