How common is masturbation?

Very. Maturbation is very common in most people.
Masturbation is real. Masturbation is part of the human experience. Taking morality out of the equation and investigating this scientifically shows us that almost all humans in all cultures, races, religions, and in every other conceivable category one finds that masturbation is common. There is no inherent issue with masturbation and health. Every human has a right to know their body.