Tracking arrows and other ammunition in a RPG stinks. Your paper gets all messed up with little tick marks, and besides, people will sometimes just stop doing it because it's such a drag.
In a one-minute combat round like the ones in the very oldest versions of D&D and CHAINMAIL*, an archer or slinger will probably take many shots. He will only get in one telling blow, like a melee guy does, but who knows how many arrows that takes? Therefore tracking individual arrows makes little sense.
For these reasons, we should look for a system that feels good but doesn't require granular arrow tracking.
What we could do is this: buy arrows by the "bundle" rather than by twenties. A bundle is whatever you can jam into a quiver. So one quiver holds one "bundle," which is the new unit of measuring ammunition. Same for a bolt case or a handful or rocks, but we can call it a quiver for consistency's sake.
Then we can say you will have enough arrows to use for the next instance of combat. But after that instance, roll 1d6. On a 1, you have run out of arrows! That quiver is empty.
If you have some ability that allows you to fire off two arrows in a round, (the Haste spell, for instance, or if you are a Royal Archer), then you are out on a 1-2 on 1d6.
The exception to this rule is for magic ammunition. Magic arrows and so forth are always individual entities and must be tracked individually, even if you have a quiver full of them.
This is a pretty swingy rule and some people might not like this level of abstraction. But I think it is analogous to rolling Hit Point damage from a sword swing or a Saving Throw or even generating a treasure hoard. So if you like those things, you might like this too.
Some people might find a 1-in-6 chance of running out of arrows after his first instance of combat to be too harsh. If you like, you could make it a 1d8, 1d10, or 1d12 roll instead.
This system can also be used for other resources that might run out, such as bandages, a crumbly old holy relic, or lock picks. The possibilities are interesting!
What do you think of this rule?