In the Workspaces section of the Window manager configuration is where you set the number of workspaces. Since you did not post your current window manager I assume it is sawfish.
run
gnomecc
and look in the section called sawfish window manager. It is also in the main menu under programs/settings/sawfish window manager/Workspaces
to switch window managers run
switchdesk
and select the one you want.
startx will bring back the last session saved on your system.