Since you only get 3 stars per level, the minimum number of levels you need to complete for 100 stars is 34. So any levels you get less than 3 stars will likely bump that number up past that. It took me 40 levels to get my 100th star. But I didn't bother with trying to max each level.
Always shoot for high scores. Here are things that help:
- Make and use power crystals (windchill, iceberg, glacier)
- Finish levels with lots of moves left
- Combo moves and cascades (side-by-side matches & matches that occur immediately after your initial match)
- Speed multipliers (make several matches in rapid succession)