In a competitive fight for the partscore, the general LOTT (Law of Total Tricks Guideline) is:
With 8 trumps, compete to the 2 level, but with 9, compete to the 3 level.
This rough guideline doesn't take into fact vulnerability (be a little more careful if vulnerable), shape (4-3-3-3 is a danger sign), or honor location (be wary of queens/jacks in the opponents' suit).
In this quiz, you can test your judgment (assume matchpoint scoring) on five 3-level decisions.