ulnar nerve-for me i get dead hands sometimes at 8 miles maybe 20 sometimes never ,take your hand off the bar put it behind you open hand and close it repeating the action on both sides,to prevent on a ride,tuck your chin into your chest and repeat chin tucks,reduces pressure at the top of spine,gloves may work,double tape on bars gets mentioned,my ring finger and little finger can feel numb for days after a long riding week,other nerves can be median and radial these affect specific areas of your hand,a quick google will give you more information,my bike has been fitted and so makes no difference for me the position of saddle
good luck