How many grafts can be transplanted in a person in his lifetime?
Simple Answer: It depends.
From my personal experience at NHI with over 18,000 hair transplant surgeries from the various clinics I have managed, the most transplanted hair grafts on a single patient done under my care was about 10,000 (over several surgeries). There are many patients between 8,000-10,000 grafts. These are often the exceptional case in which the patient had a very high density donor hair supply. Most Norwood Class 7 patients are happy with about 5,000 to 7,000 grafts.
We are born with 100,000 (average) to 200,000 (exceptionally high) hairs. Of those, only the back and side part of your scalp is considered permanent hair if you have male pattern hair loss. If you roughly estimate the back part of your scalp to be 30% of your total scalp area, then you can argue that 30,000 to 60,000 hairs in the back part of your scalp can be considered permanent, leaving with about half (theoretically) able to be moved somehow. For a typical hair transplant to be undetectable (so that your donor area scar is not noticeable) I suspect you can safely get 8,000-10,000+ hair grafts over several hair transplant procedures provided that the donor supply can be maintained healthy and scarring is minimal and able to be controlled.
Some doctors love to push the numbers. The push to high numbers are for money (to the doctor as he charges per graft), his ego (he can say he has patients with high numbers putting him in the ‘mile high club’), or the agenda of his patient. Please be careful that you as a patient know what you are getting into and best be sure that your doctor has the experience by personally meeting some of those patients who have these numbers actually done successfully.