Steps to Customize The Number of Rows of Tiles on Start Screen 8

(1) Search for Regedit on the Start screen and launch the Registry editor.

(2) When the registry editor launches, Switch to

HKEY_CURRENT_USER / Software / Microsoft / Windows / CurrentVersion / ImmersiveShell / Grid
(3) In the Grid folder, right click and select create a new DWORD(32-bit) Value option.

(4) Rename the DWORD value to Layout_MaximumRowCount 

P.S: Just copy and paste the above value to avoid errors.

(5) Now, double click the DWORD value and here you can enter any number between 2 to 5. The number here represents the number of rows of tiles you want, for instance entering 3 here will make the number of rows 3.

When you are done with this, just restart the system for the update to take place