As long as you have enough area for the hair transplants you can have as many hair transplants done as you want too or need too.
Basically the patient who is getting the hair transplants done has to have enough sufficient donor area for the multiple hair transplants.
You also need to have enough money to pay for the multiple hair transplants because hair transplants are not usually covered under health insurance because it's usually considered to be a cosmetic surgery for the hair transplant.
When going for a hair transplant make sure you get a good professional surgeon who really has a lot of experience in doing hair transplants as well as multiple hair transplants.