Two is enough — one guesser holding the phone and one clue-giver. It gets better at four or more, because the clues start competing with each other.
Does it work without a tilt sensor?
Yes. If no motion readings arrive — most desktops, and any phone where you decline the sensor prompt — the game switches to tap mode automatically: tap the bottom half of the screen for a hit, the top half to pass. Arrow keys ↓ and ↑ do the same thing on a keyboard.
Why does iPhone ask for permission?
Safari requires an explicit tap before any web page may read the motion sensor. Tap "Enable tilt controls" on the setup screen. The readings are used to detect a nod and are never stored or sent anywhere.
Can a word come up twice in one round?
No. Each round shuffles a fresh deck from the chosen pack and deals from it without replacement. If you clear the whole deck the round ends early — and that counts as a win.