Dr. Arvind Poswal from India has done more body hair to head hair transplants (known online via the acronym BHT) than any other doctor in the world. In almost all of the hair transplant cases that Dr. Poswal has presented online, it seems like body hair represent a small fraction of the total hair moved to the scalp. i.e., body hair is almost always a supplemental resource in a hair transplant. There is no hair transplant surgeon in the world who has presented as many cases online as Dr. Poswal.
At the recent ISHRS Conference, Dr. Poswal presented a very interesting overview of all that is involved in BHTs. Luckily for us, this presentation is now available online: