It is a good question you have asked
And for your question I'll have to say no. It is not harmful to health. In fact it is very good to mastrubate for your sexual health.
It acts as stress buster and also helps In hormonal balance if you do it on regular intervals but not too frequently. It has to be done 4 Times in a week. It's not that this is the maximum per weem you can go upto 6 Times a week. It helps in maintaining your stress under control and also keeps you out of frustration..so I finally conclude that there Sunni harm in masturbating.