You should train everything at least twice a week.
The body adapts to stimulus. Stimulus in the gym is weight.
Mass = Strength = ( Weight * Reps * Sets ) / Years trained
I am going to go on a bit of a rant because that split is super popular and is super straight up garbage and for some reason it just wont go away.
-2 Push days to 1 Pull and the way most people train shoulders it is 3 push days. That is guaranteed posture problems and shoulder injuries.
-Having entire workout/days dedicated to Arms or Shoulders is a wasted day unless you would normally do cardio/nothing there. It's a waste because literally every upper body movement you should already be doing trains them more efficiently than the typical exercise selection on those days. Add in that if your split is right you could have a much more effective workout here instead of focusing on 2% of your muscles.
-If you want to grow you need to train Squats, Deads and often. Your body adapts to stimulus and Squats and Deads are the kings of stimulus.
To train 6 days a week this is literally the best split you could use:
Day 1 Lower - Squats, Deads, Calves (lunges optional)
Day 2 Pull - Barbell Rows, Pullups, Biceps (T-Bar Rows, Back Flys optional)
Day 3 Push - Bench Press, Military Press, Triceps (Dips, Incline press and dumbells optional)
Deadlifts can go on Day 2 if you prefer.
For reps/sets 5-3-1 periodization is pretty good.
Or even simpler just do 5 rep sets slowly adding weight until you fail to get 5 reps. Take 5-15% off the bar and do some more sets for volume.