I have a mole in on my head. is it gonna develop to melanoma...?

There isn't a set "percentage" per se, but if you keep knicking it with the clippers there is a chance it could be cancerous down the road. You need to have it checked by a dermatologist and possibly have it removed. Your doctor will have it sent to a lab if it looks suspicious. I have mole checks every 6 months and have had approx. 15 moles removed during my lifetime (either because they got irritated by clothing, for cosmetic reasons, or because the doctor thought they looked abnormal). I had one on the back of my knee that I kept hitting with the razor when I shaved my legs. The derm said that it should be removed because of the chance it might cause problems in the future. I also had one on my back that bled when it got rubbed by my bra strap, so it was removed. At the very least, go have your mole checked by a doctor, and tell him/her that you keep hitting it and it bleeds.