Normally you don’t because the Metro Start Screen is smart enough knowing your screen resolution to decide how many rows should be put to best fit on your screen. However, if for some reason you do want to change the number of rows showing in the Start Screen, here is a registry hack for you.
1. Open Regedit.
2. Navigate through the following key:
3. Look for Layout_MaximumRowCount entry on the right panel. If not found, create it as a DWord-32 Value. Then change the value to the number of rows you wish to have on the Start Screen.
Logoff and log back in to have the change take effect.
Here is my Start Screen before I made the change, 4 rows of apps on a screen with resolution at 1366×768.
And here is after. Note that there are 3 rows of Apps instead 4.