Hi Cindy,
Trezor Model T generate a 12-word private seed by default and Model One 24 words by default, but you can specify other lengths during setup, like 18 words. See more info about it here and here.
If you haven’t put any coins or tokens into your Trezor wallet yet, and you want another seed length, then you can simply initialize it with advanced commands.
Also read about running a Dry-run recovery to test your seed.