The first time a die reaches 21, you get 1 point. The next time, you will score 2. You get the idea.
Pay attention: suppose a die has reached 21 for the 10th time. That die will not go back to zero, it will now start at 10.
I was skeptical about this game. But I think it's pretty good. I've played it about 20 times now. My high score on the classic mode is 577. Which I think is pretty good. Right now there's no leaderboard.