Only 1 workspace in Unity 2D
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 -
https://bugs.launchpad.net/ubuntu/+source/unity-2d/+bug/826089
In any event you can set the # of workspaces for unity-2d thru gconf, from a terminal you'd run this command
gconftool-2 -s -t integer /apps/metacity/general/num_workspaces 4
Alternately if you have gconf-editor installed you could open it & browse to /apps/metacity/general/num_workspaces & adjust there.