In Unity 2D there isn't any multiple workspaces. How can add more than that 1?
Update: Unity 2D now has workspaces, see this Omg!Ubuntu! post for more info.
From the Unity 2D announcement blog post:
In it’s current state, many of Unity’s features from Maverick have already been implemented (dash, places, launcher, panel) and others (uTouch, workspace switcher, accordion effect, etc) will be landing over the next few months.
Unity 2D does not yet support workspaces, but it will in the near future.
Update for GUI solution
Unity-2D with 4 workspaces (default)
Change the number of workspaces
Restart metacity so changes take affect.
Alt+F2, then type in
Your computer will have a temporary seizure, then it will be back to normal - but with added workspaces!
Unity-2D workplace-switcher using 6 workspaces