If you want more than 128 bits of entropy in your BIP39 seed phrase, then you need to use more than 12 words, with the most common alternative being 24 words, which provides 256 bits of security with an 8 bit checksum. Note that as I said above, however, your private keys generated from a 24 word seed phrase still "only" have 128 bits of security.
May be stupid question, still:
What if the private key is not generated from a seed phrase? What if the private key is just one random (may need better word here) number in the range of valid private keys? Would this provide the same 128 bits? Can't we reach 160? Or I'm confusing something?