I don't think masturbation is harmful. In fact, there's plenty of medical evidence that masturbation can be good for you. Besides, our body is constantly producing sperm so you need to release it at some point to make way for the new.
I don't think there's a set number of times you should masturbate each week because each person is different. As long as your body isn't feeling any pain/discomfort and it's not causing any disruption to your social/work life then it should be okay! Personally, I used to do it everyday (sometimes twice a day) because I'm a horny young male haha but now I have a busy work/study schedule so I do it 2/3 times a week. On very busy weeks, I'll forgot to masturbate for a week!
If you do it everyday, you'll notice that you ejaculate less and your ejaculation may be of watery consistency. That's what usually happens to me. If I save up for a few days, I tend to shoot out more cum and it has a thick sticky consistency.
I hope this helped! My answers are based on my own personal experience so I recommend you do your own research from reputable sources.