Moreover, 12 words is easier if someone wants to memorize them (never forget to make physical backups, memorizing the mnemonic is just a plus).
The passphrase is very important, because even if the generated seed has low entropy, I can increase the security of the mnemonic by extending it using passphrase.
For passphrase generation, I recommend using 6 random words or more. Random words of 6 words have, for example, the same entropy as a password like "#\KL`@cZ3W*-", I usually think that 8 or more is great.
Do not trust your brain to remember the 12 words phrases and not writing them down somewhere. There are many chances that one may forget all or any word of the phrases and you will be deprived of your money stored in that wallet. DO NOT TAKE THAT RISK.
Secondly, when you extend the phrases, it is better to add three to four easy words rather than complex words with special characters etc. Easy English words will do the purpose. Keep the things as simple as possible.