In Sea of Thieves, you can refer to the following for how many shells a Sea of Thieves backpack can hold:
The maximum number of shells you can carry is 10. The maximum number of stacks of ordinary black eggs in the backpack is 10, and the maximum number in the container is 99. The purpose of setting the amount of cannonballs carried in the backpack is to limit the pirates from staying on the cannon for output.
