How many letter tiles does one need to build any 24 word mnemonic?
Actually the answer is rather simple, Cryptosteel did a letter frequency analysis on the bip39 wordlist and found that you can represent any word in the list using only 4 characters. Therefore, for a 24 word recovery phrase, you take 4x24 or 96 characters.

Edit: Oh oops, I thought the question was that ability to store all combinations instead of the tiles provided. to which my answer is, they probably ran an analysis and decided on that number of tiles based on their data. If it bothers you, you can just use a cold wallet that includes more tiles like simbit that includes 400.