Generally speaking, as a fitness coach, there are more tests on ACE (American Sports Committee) and NSCA (American Sports Physical Fitness Association), while as a rehabilitation coach, there are more tests on NASM (American National Sports Medicine Association) and ACSM (American Sports Medicine Association), but generally speaking, as a fitness coach, there are not so strict requirements.
Generally, four certifications are enough. Ace and NASM are cheaper and better proving grounds. I suggest starting with these two. The price of ACE is around 17000.