The player with the most amount of coins invested into a windmill will get the number of stars that that windmill have. There are a total of seven windmills on the board map, each with a varying amount of stars, going up to three.
During Bowser Time, there is the possibility that Bowser may jump upon a random windmill causing it to be destroyed. When this happens, the coins invested into the windmill will be lost, and the owner will lose their stars. He may also send Koopa Kid to steal coins from a random windmill.