Spare ammo for your Crossbow or Bow tends to drop frequently so save your Inventory space and don't carry any spares. Instead, if you find that you're running low or would like to replenish your supply, you can pick up any dropped Arrows or Bolts and from your Inventory replace the new pick up with the old Ammo/Bolts then drop it. Arrows can also be replenished even if your Inventory is full.