The Bow of Plenty is one of the magical objects kept in the Black Keep. It is a bow that can conjure arrows to fire by pulling back the bowstring, however the Bow can only conjure up to 300 arrows a day.
Additionally, the wielder of the bow can declare any number from 1 to 300 and that many arrows will fly after releasing the bowstring, but that depletes the number of arrows that can be fired faster. If all 300 arrows are used up, the bow will not conjure anymore until the next day.