Windows 8 typically does a good job of automatically adjusting the number rows in the Start Menu depending on your screen resolution. The automatic row adjustment works great for up to six rows. If you screen has vertical space to spare, this guide will help you increase the maximum number of rows displayed beyond six. On test systems I have seen up to 12 rows displayed. More rows may be possible if you have access to a jumbo-tron or one of those Perceptive Pixel 82" touch PCs .
The easiest method to increase the rows is to use the tile setting menu. While the Start Menu is displayed bring up the Charms bar and select Settings.
Then select Tiles.
And if your screen will support showing additional tiles, an option to show more tiles will be visible.
Just toggle the switch and more Start Menu tiles will be displayed.