The English-language wordlist for the BIP39 standard has 2048 words, so if the phrase contained only 12 random words, the number of possible combinations would be 2048^12 = 2^132, and the phrase would have 132 bits of security. However, some of the data in a BIP39 phrase is not random, so the actual security of a 12-word BIP39 mnemonic phrase is only 128 bits. This is approximately the same strength as all Bitcoin private keys, so most experts consider it to be sufficiently secure.

A bitcoin private key is 256 bits. So you would need about 24 words (bip39 draws from 2048 words) to equal that. (2048^24= about 3x10^79 2^256= about 1x10^77)

Notice in the actual unsafe working examples below that:

1. 12 words utilizes 128 (4x32) bits of entropy

2. 18 words utilizes 192 (4x48) bits of entropy

3. 24 words utilized 256 (4x64) bits of entropy

Note the results below can be easily extended to 15 and 21 BIP 39 seed words.

% echo -n "This is a VERY pooor low entropy brain wallet" | bx base16-encode | bx sha256 | cut -c 1-32 | bx mnemonic-new

health boil host ostrich fire spike body solar collect harvest catalog crystal

% echo -n "This is a VERY pooor low entropy brain wallet" | bx base16-encode | bx sha256 | cut -c 1-48 | bx mnemonic-new

health boil host ostrich fire spike body solar collect harvest catalog cup mix tattoo market festival sudden please

% echo -n "This is a VERY pooor low entropy brain wallet" | bx base16-encode | bx sha256 | cut -c 1-64 | bx mnemonic-new

health boil host ostrich fire spike body solar collect harvest catalog cup mix tattoo market festival sudden pony notable beach wide health original inform

Also note the repetition of the BIP 39 words above.

Well for starters that should exponentially increase the security of the password if anything else