There are three main organizations with the credentials to become a certified trainer: American College of Sports Medicine, National Academy of Sports Medicine, and American Council on Exercise. Also the National Strength and Conditioning Association. However there seems to be hundreds of different certifications nowadays.
Not sure what you mean by licenses? Personal trainers are not licensed, that usually is a state controlled mandate. Are you are asking what certifying association has the largest active members? Most associations have different levels of certifications, and different specialties. I am wondering what your question is driving at?