Welcome to HealthcareMagic forum.
is not harmful , it is a natural method of self satisfaction and very commonly done. you need not worry , it has nothing to do with your under weight.
I advice you to take good food, protein rich diet, and active exercise , also check your blood with tests for anemia
, you could also benefit from multivitamins .
You cannot count your sperms with normal eyes, it needs a semen analysis
under microscope, I feel this is not needed for you right now .
Wish you good health