Playing our second introductory scenario we had this interesting cards combination: Box barrage + Artillery Bombard + Shell Shortages. The question is very simple: Does Artillery Bombard and Shell Shortages apply to both the attacks or only on the first one? In both card we read the word "turn", so we applied the effects on both bombardament.
Again, the best way it to read the cards literally.
So "Shell Shortage" text is key here(emphasis added intentionally):
Play before your opponent's reserve artillery combat roll.
Target opponent's reserve artillery.
Opponent's reserve artillery targeting roll dice are reduced by 2 this turn
So simply speaking, opponent’s reserve artillery targeting roll dice are reduced by 2 dice this turn. You played well!
BTW, should you need more materials for TGW game please have a look here - this is great page about that game:
my reading is that you played this correctly, all effects are per turn.