if the phrase contained 12 random words, the phrase would have 132 bits of security, so the actual security of a 12-word BIP39 mnemonic phrase is 128 bits as regards a Bitcoin private key security too. That's why some experts consider using only 12 words.
A bitcoin private key is 256 bits, to make up that bit you would need about 24 words to equal that and that's why some experts also consider to use 24.
So either 12 or 24, they all same bit of security as Bitcoin private key which is 128.
Also is higher the number the better with security? I read somewhere that if you can get 6 out of 12 recovery seed you may be able to get the rest, is this true? I just want to know how far wallet security can go.
If you read this thread https://bitcointalksearch.org/topic/trezor-12-or-24-word-recovery-5205380, you will get some answers about 12/24 seed phrase security. Even if someone knows the 6 words of your phrase they will only end up importing a different wallet unless they have full access of your whole phrase.
