There are three figures to keep firmly in mind when bidding, to wit:
Twenty
Twenty-six
Thirty-three

I have seen others give the key figures as 26, 29, 33 and 37. Twenty-nine was said to represent the number needed for a minor-suit, eleven-trick game, and 37 the number needed for grand slam, but I have never seen those figures become operative, as it were, or take on significance. On the other hand, I include the figure 20, which I have never seen anyone else give on a par with 26 & 33, because that number takes on importance quite frequently. Why it's important that you heed it and dangerous to ignore will be discussed on the page with that heading.