There's no easy way to modify the number of workspaces in Unity - there is an option in Gconf but that's not exactly user-friendly -, so MrChrisDruif has created an Unity icon (with quicklists) to easily add or remove workspaces rows/columns.
To download and copy the required files to the right locations, copy/paste the following commands in a terminal:
wget http://webupd8.googlecode.com/files/workspaces.tar.gz tar -xvf workspaces.tar.gz chmod +x workspaces-shortcuts mv workspaces.desktop ~/.local/share/applications/ sudo mv workspaces-shortcuts /usr/local/bin/
Then, open Nautilus and navigate to ~/.local/share/applications/ (".local" is a hidden directory in your home folder so press CTRL + H to see it) and drag the "Workspaces" icon to the Unity launcher.
Credits for the script/quicklists: MrChrisDruif @ AskUbuntu.