The Viewport Settings Window is used to change the grid display in the model viewports. Select Viewport Settings from the Main Window's View menu.
The Default Grid Unit field specifies the default distance between each grid line. There is a separate setting for the canvas and 3D perspective viewports. The number of actual lines drawn depends on the zoom factor of the viewport (the grid spacing doubles or halves as needed).
For the perspective viewport you can specify the number of grid lines that are drawn (in each dimension) using the Grid Lines field. You can turn grid lines on and off for each plane independently using the Plane checkboxes.
Press Ok to keep your changes or press Cancel to ignore any changes.