Answer:
When we do a scale model of something (like a building, a house, or whatever) al the properties of the original thing must also be in the model.
So for example, you want to do a model of a house, and in the backyard of the house there are 4 trees, then in the model of the house you also need to put 4 trees in the backyard (indifferent of the scale of the model).
Then the number of boulders in the really fountain should be the same as the number of boulders in the scale model of the fountain.