Most ammunition in DW have "ammo" property — it is usually a small number, like 3-4:

Bundle of Arrows 3 ammo, 1 coin, 1 weight

The tag description says this number "represents what you have left on hand", whatever that means:

n Ammo: It counts as ammunition for appropriate ranged weapons. The number indicated does not represent individual arrows or sling stones, but represents what you have left on hand.

The only other mention of ammo is the Volley move:

You have to take several shots, reducing your ammo by one

So, when you reduce your particular "ammo" to zero, are you out of ammunition and can't fire at all, or you just can't choose to mark off ammo on a 7–9?

For comparison, using throwing weapons suggests to "choose from the other two options":

If you’re throwing something that doesn't have ammo (maybe you’ve got a move that makes your shield throwable) you can't choose to mark off ammo. Choose from the other two options instead.

...it also specifically says what happens next:

once you throw it, it’s gone until you can recover it

When you use up your ammo, you're out of ammo.

You can Volley with a ranged weapon if you "have ammunition" for it. But Fletcher's Volleys, even when they don't consume ammo, are not necessarily with exactly one arrow, any more than Shanksworth's Backstab involves exactly one stab or Fightgar's Hack and Slash contains one hack and one slash. Being able to Volley means that, in the story, you have enough ammunition.

Tags exist to tell you how equipment plays into the story:

Each piece of equipment will have a number of tags. These will tell you something about how the equipment affects the character using it (like +Armor) or suggest something about the way it is used (like the Range tags). Like everything else in Dungeon World, these guide the fiction you’re creating in play. 

When you drop to 0 total ammo, for story purposes you no longer have enough ready ammunition for a Volley. That's because the numbers in the ammo tag are also there to mean things in the story, and 0 means 0.

It's possible to get enough ammunition for a Volley without getting 1 ammo! You just have to work out the fiction that goes from "I'm out of ammo" to "then I find some" to "and keep on firing". If Fletcher's got no arrows left in his inventory but is out on a hilltop where a bunch of friendly archers did that "stab arrows into the ground for easy access" thing, then he has fictional access to enough ammunition to Volley, but can't spend points of ammo for the purposes of moves that care about that. If Shanksworth is fleeing down a cobblestone road it probably isn't too big an ask to find a rock big enough to sling without breaking stride. If Stringfellow has only a bow to stand against a beast swooping out of the night, well, pray to lady luck and search your pockets, bard. And hurry.

Tags like “ammo” are an abstracted way to keep track something without having to keep explicit count. Your quiver having 3 ammo doesn't mean three individual arrows. Moves like Volley may make you decrement this abstract value as, in the fiction, you are skirting the threshold of running out of arrows in your attempt to make an effective attack because, mechanically, you rolled a 7-9 result.

Having zero of something means you've run out. You've expended your reserve of arrows and can no longer, in the fiction, shoot them.

The Ammo Tag is defined as follows:

n Ammo: It counts as ammunition for appropriate ranged weapons. The number indicated does not represent individual arrows or sling stones, but represents what you have left on hand.

When you check the last box, you're down to 0 ammo left on hand. When you run out of ammunition on hand, you no longer have anything to shoot from your bow. With nothing to shoot, you can no longer trigger the Volley move in the fiction.

Compare this with a similarly formatted Tag Uses:

n Uses: It can only be used n times.

When you're out of uses, just like when you're out of ammo, the fictional positional of the equipment has dried up and you must find more before attempting the Move again.

The best answer that I can come up with for this is...

It Depends on What You're Playing

and

It Depends on Your GM/DM

And I don't mean that in that "oh you're playing DnD vs Pathfinder", no.

It depends on what KIND of game you're playing, and who you're playing it with. 99% of the time, ammunition in games won't be counted, not because you don't need it, but because it's distracting. High Fantasy Ranger is fun to play, but not if you're going to have to keep track of whether or not an arrow breaks after you fire it, or if it can be re-used on a roll, etc.

Most GMs/DMs assume you're keeping ammunition stocked every time you enter a town. (It's just assumed your party stopped by a general store and stocked up on whatever you were low on). Ammunition is effectively so cheap in most roleplaying games it's assumed you don't actually have to think about it as a purchase at all.

This is not a rule

Expect ammunition and food and water to be counted if you're playing some sort of survival game or campaign. An old friend of mine who told some truly epic stories used to not count anything, and then, whenever we'd arrive someplace without ready access to something, would ask us. "[Character Name], how much [whatever] do you usually carry on your person?" And we'd defend our number, but it was never actually counted until there was a reasonable assumption that you'd RUN OUT.

-

EDIT::Of course, this didn't answer your main question at all. I realized after I finished. The correct answer to your question is (RAW):

You can't mark off ammo on a 7-9

The reason for this is that you don't actually lose any ammo when you're just using your weapon, it's only on a bad roll, implying you're picking up your arrows after, and the breakage isn't enough to actually lower your ammo count by much. There's also this:

If you’re throwing something that doesn’t have ammo (maybe you’ve got a move that makes your shield throwable) you can’t choose to mark off ammo.

This sets a pretty clear precedent on the fact that there are places where you won't be able to choose that specific section.