I believe it would depend upon the entropy used for the seed, as that would affect how many bits the checksum represents. https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki#Generating_the_mnemonic

So for a 24 word / 256 bit mnemonic the checksum is 8 bits, thus there are 8 valid checksums and if you picked a random checksum from the wordlist of 2048 words you'd have a 1 in 256 chance of picking a valid one.

On Sun, Dec 23, 2018 at 1:44 PM Aymeric Vitte via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
Has anybody already looked at this: given N randomly chosen words
belonging to a BIP39 2048 words dictionary, what is the probability to
get a "valid" BIP39 seed (ie with the right checksum)?

The result looks (very) surprising to me and might have some use cases,
just would like to know if this topic has already been discussed before
going further

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