Basic ruler guides (grid) on a master are created automatically when you create a master. However, you can modify the basic grid at any time.
IMPORTANT: changing grid on a master will change it for all pages that share the master.
Modify basic ruler guides (grid) on master
1.Open the master of the spread or page you want to modify grid of.
2.On the Layout menu, point to Masters, click Setup, and then change what you want.
Using presets
Under Preset, select a preset.
Note: you may then change - if you want - any of proposed values. Changes won't affect the preset itself.
Change number of columns
Click Set under Columns, and then select number of columns - the maximum number of columns is 12.
Notes
Have in mind that the script commands that read grid values (e.g. column width, module height) read only from the master page data, and only from the values that are set in Layout > Master Setup. Therefore, if you manually move the guides on a master, all the scripts that use the column width and the module height reading from the master data will malfunction. DON'T DO IT!
See also
Create additional ruler guides on master
Create/modify basic ruler guides on layer