Just as Upgrade00 pointed out. The security is actually on the number bits of entropy produced and the best is that which is between 128 bits and 256bits. A words less than 12 will certainly generate a lower entropy which will weaken the strength of guessing them. The 12 words I think produces the same number of bits as a private key (128) which in terms of security doesn’t matters how many bits of entropy is generated, since the level of the private key security is 128 bits. That is why most wallet stick to the 12 words.
Read on here
https://learnmeabitcoin.com/technical/mnemonic#bip39-wordlistIt keep pondering on my mind in asking why are some seed phrase being generated on different number range and why is 12 and 24 the most common numbers of seed phrase.