B2 p209 is just an overview explaining the terms, no quantities of anything is given.
The procedure for buying cargo is on p221, the available quantity is given by p220 as you note.
The Cargo ID can be determined as per p210.
It looks fairly similar to previous editions, and a well-defined procedure?
Agreed, the rest is fine, just cannot see how to calculate the cargo quantities. I seem to recall in previous editions various sized lots being available. Perhaps the intention was simply that, as a specualator, you can just buy up to 100 tons cargo each day, until the hold is full, although each lot, whilst having the same trade descriptor, they will have differing randomly created details?
regards