There seems to be an issue with the Horizontal Thumbnails when a Button Style greater than 18 is chosen. (I tested this with an existing project and 2 new projects, one with and one without a blank template)

For Button Styles 1-18 you get this effect as expected (a single row with the requested number of thumbnails).

But for Button Styles > 18 (with 10 photos in the gallery) you will get this effect with no navigation buttons.