Personally I find the goal is working out how to bleed as much as you can from a mechanic, theme or setpiece and showcase it within a level. Then do the same with another mechanic, theme or setpiece in the next level. Do this until you have exhausted the toolset/your pool of ideas without repeition and you'll end up with a good set of levels.
For reference, in the RGM jam using the toolset available to me, I felt that (given the number of assets and ingredients I had created) the sweet spot was 4 levels. However, this tooset seems a little more advanced, so I feel it's very much down to how much you lean into the unique mechanics available/how many additional features you script.
