u/boodytaymour

Pretty sure this question got asked a lot, but since i started going to the gym (6 months), i've been training 7 reps (failure) with heavy weight, started seeing people on the internet saying back/shoulder exercises must be 10-12 reps, is this true? and based on what do people say a muscle should train how many reps, anybody knows?
PS. im skinny trying to gain muscle if that is relevant.