You'll always stand on any hard total of 17 through 21.
This one's pretty simple. If you have a 17, there are a really low percentage of cards that won't make you bust. Aces, twos, threes, and fours will help your hand, but all of the other cards will hurt you.
Those cards make up 16 cards in the deck. Which means there are 36 other cards in the deck which will make you lost instantly.
And that's for a total of 17. The ratio gets worse when you look at a hard 18 or 19.
And of course, a total of 21 is a winning hand.