The number of stars awarded in battle depends on your performance.

Here's a breakdown:

1. Winning the battle will get you 1 star.
2. Winning and maintaining at least 50% of your HP at the end of the battle will get you 2 stars.
3. Winning and maintaining at least 70% of your HP at the end of the battle will get you 3 stars.

Please be aware that these HP requirements may fluctuate as you progress between areas and maps. 

These figures only serve as a guide to understanding the concept. To track your progress and requirements, open an island, and you'll find the Star progress bar at the bottom of the page. Simply tap on the "i" button to access specific requirements.