According to https://en.bitcoin.it/wiki/Private_key#Range_of_valid_ECDSA_private_keys, not all 2256 bits numbers are valid private keys, what about mnemonic phrases?
According to BIP39 standards, we can use around 2048 words to generate a mnemonic key. For us to generate a 12 word key pair the combinations could range between 204812 which could be simplified to 2132 and the mnemonic would eventually have 132 bits of security. But due to some randomness, the security could be around 128 bits which is equal to a private key security.But generating a word yourself from the 2048 lists is pretty dangerous when security is concerned. Allow your wallet to work for you. For a complete understanding of BIP39 conventions you may read this
