On my Galaxy S9+, I have 4 buttons: power, vol up/down and a stupid useless bixby button. Ignoring how to put it into 'password' mode, each of these buttons is effectively a bit. I assume you need at least 1 key to register an input (otherwise wait for a pause which I won't consider), so you get 15 possibilities. I think you could make a reasonably strong password using 4-8 chords (15^4-15^8 possibilities). Anyways, it's certainly better than those swipe passwords with an effective length of 9 Cr 4 == 128 which are readily seen and copied by nearby people.