To change the number of positions needed at a given shift, first click on the "Edit shift" button (it looks like a pencil icon) for that shift:
In the pop-up dialog, you can either use the up and down arrows or type directly into the boxes to change the number of volunteers needed in each job at this shift:
Note that shifts can only be edited from the Edit Schedule page. If you are viewing a published schedule or a specific opportunity in the Opportunities pane, click on the "Edit" button to access the Edit Schedule page for a given schedule.