2048^12 possible seeds
isnt chances much lower because of you only need to match few letters to know word?
Each word had a unique first 4 letters, but that doesn't change that there are 2048 different words, and each word encodes 11 bits of entropy.
Bitcoin private keys themselves only have 128 bits of security, so anything beyond that for a seed phrase is technically unnecessary. Further, regardless of how long your seed phrase is, it is passed through the exact same process to create a 256 bit master private key and a 256 bit master chain code. Therefore, a 12 word seed phrase is just as secure as a 600 word seed phrase.