They are not to be confused with the seed words used by HD wallets [2], based on the BIP-0039 wordlist [3]. For this standard most wallets use 24 words and upwards, which should be sufficiently secure for the foreseeable future.
Minor correction... BIP39 specifies that:
The mnemonic must encode entropy in a multiple of 32 bits. With more entropy security is improved but the sentence length increases. We refer to the initial entropy length as ENT. The allowed size of ENT is 128-256 bits.
As it must be in multiples of 32 bits, this equates to a mnemonic sentence length between 12-24 words in 3 word "steps"... so 12, 15, 18, 21 or 24 words.
Ah, good catch, thank you for correcting me.
I was thinking about hardware wallets and for some reason I misremembered Ledger using 32 seed words as opposed to Trezor's 24 seed words Turns out Ledger
uses 24 seed words and it's just the custom passphrase that is longer than Trezor's (100 characters [1] vs 50 characters [2]).