I'm running on a 1366x768 resolution
This resolution supports 4 rows of tiles.
The 'Shows more tiles' option only appears if your resolution allows for more than 6 rows. Once you passed 6, you can choose to have 6 maximum, or as many as the screen can fit.
What I was talking about was the thumbnails in the photos app, which shows 3 rows of thumbnails, despite the old app (which had the same size thumbnails) showing 4 rows on the same resolution.
The only setting available is 'Shuffle photos on the Photos tile'

