So, if there is a thing that people are supposed to do, and they don't do it, and you think it adds to the play of the game, it's on you as the rules hacker/designer to make it easy. I think as long as people are buying arrows in lots of 12 or 20 they should pay attention to depletion. But until now I haven't come up with an easy rule that makes some kind of sense.
|So happy, sucking up all your ammo.|
1. If an arrow or crossbow bolt does damage but does not kill its target, cross it off; in effect it gets stuck in there and broken off by the still-alive creature. Sling bullets are hardier, so do not suffer this fate.
2. If the party flees the scene of a combat without time to rest and pick up missiles, anyone who shot missiles crosses off two if the fight was short, five if the fight was long, and more for epic fights. If a player thinks this is too many, they are free to keep track of their missiles one by one.
If your players are marking down TWELVE ARROWS or TWENTY ARROWS IN QUIVER, they can just cross off letters from those phrases as ammo gets depleted. Same goes for TWENTY BULLETS IN POUCH and TWENTY QUARRELS IN CASE.