If you just installed compiz-fusion, or if you recently upgraded to Ubuntu 7.10, you may notice that where you previously had four workspaces, you only have two. There’s that nice right-click option in the workspaces are the normally would fix this issue for it (Workspaces, Rows, Columns) but for some reason, that’s patiently ignored.
Here’s how you fix it – open up the Compiz Configuration Settings Manager, either by choosing System->Preferences->Advanced Desktop Effects Settings, or from the command line by running “ccsm”.
Go to the General Option section at the top, and then the Desktop Size tab. Change the “2” in the Horizontal size field to a “4” and you’ll be back to your original standard number of workspaces.