In Unity, the default windows manager in Ubuntu 11.11, there is no obvious way to change the number of available workspaces/virtual desktops. This is how I did it:

1. Go to the Dash Home and search for CCSM.
2. Open CCSM - Compiz Config Settings Manager
3. In the Compiz Config Menu select 'General Options'
4. Go to the 'Desktop Size' and change your desktop settings by adjusting the number of vertical desktops.