Some items require ammunition—arrows, bolts, bullets, charges, etc. But in the heat of battle, it's not always clear how much ammo you have left. Instead of tracking each individual shot, take a d12—this is your Ammunition die. Roll it whenever you take a shot: if you roll a 1 or 2, the die gets one size smaller. If you're down to one piece of ammunition and you use it, that's it—it's all gone! Remove it from your inventory.

Citation: https://giffyglyph.com/darkerdungeons/grimoire/4.0.0/en/ammunition_dice.html

Ammunition Die

d12 → d10 → d8 → d6 → d4 → 1

Average uses per die size