When you find crossbow bolts you find them in a bulk, which is equivalent to a pack. These can be ejected, and you will get 1-5 bolts.
When ejected, this stack of bolts can be loaded back into the crossbow, which shows 1 bolt. If the bolt is dragged out into inventory, it's a bulk again
Now if you press "f" or scroll you can select reload with: the bulk, like if it was a quier.