How do I change the number of desktops in Mountain Lion?
Open &quot;Mission Control&quot; (icon should be in the Dock or else through Launchpad) and move your mouse to the upper-right hand corner. A '+' sign will appear allowing you to create an additional virtual desktop. The dimmed half of a new desktop appears in the upper left if the dock is changed to the right side of the screen but otherwise is in the upper right when the dock is in the left or the bottom.
Add a Desktop

Remove a Desktop