Can a lot of masturbation cause the penis shaft to go numb ?

IT CAN: It can temporarily be desensitized with recurrent manipulation.

Maybe but...: If it has, then i commend you on your perseverance. However, a pudendal neuropathy (which this may well be) is often caused not by local pressure but by pressure on the pudendal nerve in the perineum (skin between legs). A common cause of this is a lot of bicycle riding without padding. I suggest you temporarily curtail your domestic recreational activity. If you don't get better, see your doctor.