Ledger phrases are generated following BIP39 standard
Its basically a library of 2048 unique words sorted in alfabetic order.
Therefore generating phrase by pickin 24 words at random gives you 2048^24 = 2^264 combinations,a number so astronomical there is no realistic chance of two wallets/people randomly ending up with the same phrase.
However because (as you noted) 12'th and 24'th words are checksum, the number of variations drops from 2^264 => 2^256 ,yet that is still more than a number of atoms in the observable Universe.
The number 2^256 is likely something you have already seen A LOT of time, because it's used in EVM constantly.
Reason for that is that Ethereum private key has 256bits => 2^256 possible combinations too. In fact, your recovery phrase is simply your
private key writen in a different (more readable ) format.