This is an odd topic. Similar to the rule of thirds that so many know there is another good rule to know and follow. This is the rule of odd numbers of “things” in the image.

With larger groupings it can start to get lost in the idea of "plenty".

Having a number of subjects in the frame that is odd instead of even can have some benefits not already obvious when composing. It can bring a sense of balance to an image especially with groupings of 3. This can also be felt with larger odd numbers, but it is most prevalent with the smaller groupings.

The eye tends to wander towards the center of a grouping of three. If you have something you want the viewer to focus on this can be effective at directing their attention. Having the three objects in the composition create a line or triangle is effective at creating this interest.

Odd numbers can also make the viewer feel more relaxed and comfortable. Unless your images are meant to create a feeling opposing this mood it is best to keep odds in mind. The visual harmony of these subjects in context when done properly can create a sort of peacefulness in the photo.

Lastly, odd numbers of things together is going to be more challenging to the viewer to piece together. Even numbers are easy to organize and group together. Symmetry can be a strong composition technique, but it can also be simple and boring too.

Like any good photography rule there are exceptions. Plenty of images with just two items as a focus or that draw the eye in can be delightful. These are just some suggestions on what is a good practice in composing imagery in the field.