There are at least three ways to have Outlook show more thumbnail calendars:

-- Choose View, Arrange By, Show Views in Navigation Pane to toggle that
option off. When views are not displayed, you should have room for two
calendar thumbnails, arranged one above the other.

-- Drag the vertical separator bar on the right of the navigation pane
toward the right. When you make the navigation pane wider, Outlook shows an
additional calendar thumbnail next to the first one.

-- Drag the vertical separator bar on the right of the calendar display
toward the left. This will display the TaskPad on the right side of the
screen. You can adjusts the number of calendar thumbnails displayed in the
TaskPad by dragging the horizontal separator bar above the TaskPad up or
down.