Just because ship can carry nominal value of 15K TEUs, it is very hard to estimate how many containers are currently on board. Also, empty container looks exactly like the full container from the outside, looking at the ship.

One very interesting aspect of ship operations is the art of stowage of containers. The complexity associated with it is substantial. 3D large scale Tetris. This is a good introductory article https://www.shippingandfreightresource.com/container-stowage...