Bip39 has limits on how long nmemonics can be (and they're set to be divisions of 3). I don't know how trust wallet generates seeds or what length they'll be but it makes sense it'll only give you 12 word seeds (that's 128 bits of entropy and an already impossible number to crack/guess).

Most wallets that incorporate bip39 go with 128 bits (12 words) or 256 bits (24 words). The best way to ensure your wallet is more secure is by extending these words with a passphrase that you'll remember (it can't be recovered) if Trustwallet supports that or finding a different wallet that does.