Every Location has a limited number of Minions it can hold.
That number is shown over an explosion background next to the Location name.
If the number of Minions in a Location is higher than the limit, that Location is lost for the rest of the game.
The limit is based on the total number of Minions present, not the number of colors of Minions.
Minion Effect tokens and the Villain do not count towards the limit.
If any Locations have exceeded their limit, follow the steps in Losing a Location.
In the picture shown here:
3 minions = OK
4 minions = LOST