The number of windows that should be opened for evaporative cooling depends on the size of the room and the amount of air circulation needed. Generally, it is recommended to open at least two windows to allow for adequate air circulation. If the room is larger, more windows may need to be opened to ensure that the air is properly circulated. Additionally, the windows should be opened in opposite directions to allow for cross-ventilation. This will help to ensure that the air is properly circulated and that the evaporative cooling system is working efficiently.