With Ubuntu 13.04, you certainly found out that there is, by default, only one workspace.
If you want more, there are solutions.
First of all enable workspaces, then change (if you want) the number of workspaces.
Let's see this in this tutorial.
Click Settings > Appareance > Behavior > Enable workspaces.
You have now 4 workspaces.
But if you want more you have to install ccsm, the CompizConfig Settings Manager.
Let's add it to Ubuntu:
sudo apt-get install compizconfig-settings-manager
Once downloaded, write in your shell the following command:
It should open.
Now go to General > General Options > Desktop Size.
There, choose the number of horizontal and vertical virtual sizes you would like.
You can even change the number of desktops.
Another option to add by default to Ubuntu for future version.
Anyway, you have now the choice to use the number of workspaces you want.