Hello, Masturbation is very normal and their are certain myths attached to it. Having sexual activity does the same thing. You can do as many times as you get pleasure but examine yourself that you don't have compulsive masturbation or you are not masturbating because you have persistent, recurrent, irresistible thoughts regarding masturbation. If you want to control the urge remember these urges usually come for minutes if you will involve yourself in some other activity the urge to masturbate will go. No it don't cause back pain.

Anything excess can be harmful. There is no clear number of times after which masturbation becomes harmful. If you feel that it is hampering your daily routine, then you might need help. Else there is no problem. Masturbation usually doesnt cause any physical problems per se. However if it disturbs your studies and you wish to control your urge, it is recommended that you consult a psychiatrist.