Rule of Odds
The Rule of Odds states that having an odd number of objects in a picture is more appealing to the eye. They eye tends to be more comfortable with images that contain an odd number of elements rather than an even number. If there is an even number of objects, your brain will have to organize them and the picture will become symmetrical and dull. As an example, I put two photos here. One has three kittens and the other photo has 4 kittens. In this photo, you tend to look at the subject, all three kittens.
In this photo, you tend to see the photo from a symmetrical view and you find the photo dull and boring because of that. You organize the kittens in to pairs and see the symmetry.