The idea of containing a huge amount of arrows in the backpack is good but there is a problem:
while you can stack multiple items of the same kind of arrows in a single slot eg. 4x30=120 arrows, you can shoot only 30 of them before all the others disappear.
So even if the counter under your long range weapon says eg. 120 so 4x30 arrows, after you shoot the first 30, all the other 90 remaining disappear.
it seems that the consumption of arrows is linked not only to the number of them but also on the backpack slot used to store them