The best answer would be, as much as possible!  Depending on the quality of your donor site, the maximum number that you can transplant in one mega session would be ideal to have the best density and help restore your previous hair appearance.  The limitations depend on many factors – financial, quality of the donor area, current and future needs of the patient.
We can assess the donor site for the supply to the required area.  We can detail for you the maximum number of hairs that can be transplanted to maintain a normal appearing donor area, one that will remain dense as well, without having any obvious deficiency.  Our surgeon will explain to you approximately how many hairs you may need to obtain a natural appearing result.  Many patients require more that one session, depending on how many hairs we can densely pack into the recipient site, as well, many patients will do a smaller number of hairs due to financial reasons, and then have another procedure at a later date when they are able to increase the density in the area.
It is important to realize that hair loss is usually progressive and as the years go by, you may lose the native hair surrounding the transplanted hair.  This means, at a later time, possibly many years later, you may need another hair transplant to replace these hairs.  
The transplanted hairs will have a life time of well into your 70's.  Hair that is transplanted is from the genetically programmed area at your donor site that is resistant to DHT, and will therefore not fall for many, many years.

While assessing patients, we map out the current and future needs.  With this information along with your goals and expectations we can make a plan specific to your situation.