In case, like me, you are confused by units, "23000 TEU" refers to the number of 20 foot containers they can carry.

The average value of a container is thought to be somewhere north of $50,000. Even allowing for a fair margin of "it's a bit more complicated than that", you're in the region of $100,000,000 cargo-value per ship.

And in practice they carry 40-foot containers. So 23,000 TEU is probably 11,500 containers.

In practice you can stack 40-footers as you wish, or stack 20-footers as you wish. But you can't mix them easily. See, two 20-footers are 'broken' in the middle, and will stress a 40-footer in the (unreinforced) center to the point of failure if stacked high enough. Which of course they do.

Well, if you can stack 40-footers on top of 20-footers I suppose that would be an option? That would of course make loading more complicated, but it's hardly an insurmountable obstacle, no?

Once, on the bottom layer, you can have 20-footer(s). Once you go to 40-footers you can never go back in the stack. They're stacked 10-high above the deck. So you can have <10% 20-footers. Or put them all on the bottom, and only switch to 40-ft once.

Quite, that's what I mean. Stack all the 20s first, then top it up with 40s.