Minecraft Dungeons:Quiver

"QUIVER – Tells you how many arrows you have left to loose on the wandering mob masses."

- Guide to Minecraft Dungeons: A Handbook for Heroes

When entering or returning to the camp, the arrows in the hero’s quiver reset to the default amount for the bow that they have equipped. If the hero’s currently equipped bow changes to another, the quiver is set to the appropriate amount for the newly equipped item. Currently, the only way to refill the quiver slot while in the camp is to activate the Surprise Gift enchantment and create an arrow bundle. Heroes can also use the Flaming Quiver, Torment Quiver, or Fireworks Arrow artifacts to acquire special arrows to shoot.

Arrow bundle
An arrow bundle is a consumable $$. When consumed, it adds arrows to your quiver. The amount added depends on the ammo value of the hero's bow, and where the bundle came from. Arrow Bundles dropped from enemies and normal chests give half the quiver's starting amount, bundles from supply chests give the quiver's starting amount, and bundles from the Surprise Gift enchantment give one fourth the starting value.

Obtaining
Arrow Bundles are mainly obtained by defeating mobs and opening chests.

They are a guaranteed drop from supply chests.

The common armor enchantment Surprise Gift has a chance to create Arrow Bundles, as well as other consumables.