I think the aspect ratio of most photos in posts ranges from 14:9, 5:4, 3:2 or a square.
Of course it depends on the size of the screen we’re using. So each of us will see different things. On my 13" laptop, it seems it works the best when 4 photos are showed at the same time. 2 or 3 photos tend to make into a narrow banner format, and crop too much detail from the photos.