I use Samsung Galaxy S9+, My home screen grid only allows till 5*6 grid. Is there any way to set customise grid layouts?
You can try using another launcher. Nova Launcher for example allows to set 6x6 grid. After installing, long-press on the home screen, select settings. When the settings open, click Home Screen -> Desktop grid, set it to 6x6.
There are other launchers that can also do it. Just search on your favorite app store.