Its actually only on one hand! And he uses that hand with the normal nails to stretch him out. Not that i looked closely or anything..
a follow up to Reaper's comment, it's a good thing u noticed that CUZ it's a point of character development for Jaeshin!!
Light spoiler?: They actually hurt, and Jaeshin won't use his long nails anymore, he will mention it next time that he knows it hurts, and even later he will get rid of it on his right hand (which is an important character development, because it's a status symbol for the ghosts)
jus started this (on chap 5 lol) but how do those long ass sharp fingernails not hurt or rip up his insides that shit looks so painful