Brawl Stars currently features 6 game modes, but you can’t access all of them of the all. There is a rotation that chooses the game modes and battlefields, giving you up to 4 choices at the time.
Each rotation lasts 24 hours.
There are currently 4 slots available, and each one can contain one game mode, so you will get a new game mode unlocked every 6 hours.
The 5th slot is a Special Event that occurs on weekends.