Settings -> Personalization -> Start -> Show More Tiles
If that doesn't work, then you need to go:
Settings -> System -> Display -> Size of text, apps, and items on this display. Just bring the slider to a smaller size, then you should be able to add more tiles. Depends on your device's default display.
Looks like on my 830 the size needs to be at 175% (default/recommended was 200%) in order to use 4 rows of tiles (and 5 action tiles in the notification center).