Paintings that I have done which are most successful are the ones that have no more than three major elements in them. Each element can have multiple trees, buildings, umbrellas, etc. in them but for my taste they must be grouped as one. If the one element is a set of buildings they must be connected or grouped in some way either by light, shadow, color, or a physical connection of touching each other. In any case they must all look and feel like one large block.
Sky and water can become one element as they can share the same value and color. Trees and grass can do the same but the better paintings have a unity of oneness about them and much of it is obtained by this grouping of objects.