There are two rulers. The horizontal ruler positioned above the canvas which marks out the x-ticks, and the vertical ruler positioned to the left of the canvas which marks out the y-ticks. The gap between tick marks can be changed using the Settings->Grid->Grid Settings menu item.
You can show or hide the rulers using the menu item Settings->Show Rulers. You can adjust their size and number format using the Borders tab in the Configure User Interface dialog box. When the rulers are visible, the grid unit is displayed in the corner between the rulers. You can double-click on this corner to open the grid settings dialog.