One of the frustrating experiences with Unity in Ubuntu is how hard it can be to change some configuration or settings. For example, Unity offers no direct way to change the number of workspaces in the desktop. It comes with a default of 4 workspaces arranged in a 2-by-2 grid.
To change the number of workspaces, you need to install the Compiz Config Settings Manager (CCSM). Open up CCSM, go to General Options > Desktop Size and set the Horizontal Size and Vertical Size to change the number of workspaces.
Tried with: Compiz Config Settings Manager 0.9.5.92 and Ubuntu 12.04 LTS