Ammunition Dice
Mistveil → Rules → Ammunition Dice
As an alternative to tracking each arrow or other piece of ammunition, ammo is tracked via the use of an "ammo die" which is rolled each time you use a piece of ammunition. The starting size of the die depends on the storage used for the ammunition (an arrow quiver that holds 20 arrows would start with a d12, where as a crossbow quiver that only holds 10 bolts would begin with a d8) and each time it is rolled, a result of 1 or 2 reduces the size of the die by one step, following this order:
d12 → d10 → d8 → d6 → d4 → 1
If you are down to a single piece of ammunition and you use it, there's no rolling to be made, it is gone.
In order to replenish your ammunition dice after a battle in which it was reduced, you can spend at least 1 minute recovering ammo from the battlefield once the fighting is over. Roll a d% and on a 50 or higher, you manage to recover enough ammo to restore your higher die. Alternately you can buy, craft, or find new ammunition as per normal.
Any ammunition that is unique in some way or that costs at least 1gp each must be tracked individually as per normal.