I'm using Unity 2d.
When I click on Workspaces, it zooms out, but the screen is black where the other 3 workspaces should be. When I try clicking in the black area, nothing happens.
By default the # of workspaces would be 4 & typically nothing would change that setting on it's own.
However if you did happen to login to gnome-shell, - when exiting GS it writes to gconf the # of workspaces open at exit which many times is 1. This then would affect the # available to unity-2d.
Here's a bug on this behavior -
In any event you can set the # of workspaces for unity-2d thru gconf, from a terminal you'd run this command
Alternately if you have gconf-editor installed you could open it & browse to /apps/metacity/general/num_workspaces & adjust there.