When creating or editing a Role, you can set the default Minimum and Maximum required Staff for said Role. The Minimum is the mark you have to hit to successfully staff a Shift and the Maximum is the cut-off. The Minimum will be displayed beside the Role name in the Schedule and will be the denominator used to identify under-staffed Shifts.
You can edit the default Min/Max by selecting the Role from the Roles tab and choosing 'Edit Details' from the 'Take Action' button.
To give you more precise control over the Minimum and Maximum settings, you can adjust this on a Shift-by-Shift basis. This will override the default settings depending on what you choose for the Shift itself. For example, in the image below, I only need one Shift Supervisor at any time during the chosen Shift and the settings reflect that.
You can edit Min/Max per Shift by selecting them individually from the Shift pop-up, by editing them all at the same time with the 'Edit All Role Settings' button, and in bulk with the 'Take Action' button when working with the List View of the Schedule.