Viewports and desktops are not the same concept. For technical reasons, changing the number of "desktops" or "workspaces" in environments such as GNOME, KDE, and Xfce is not the same as changing the number of viewports in Compiz. You can have multiple viewports on one desktop.
I know how to use the shortcuts in Desktop Wall to switch between viewports on a single desktop.
How do I switch between desktops?