Use the command:
gsettings set org.compiz.core:/org/compiz/profiles/unity/plugins/core/ {hsize | vsize} n
For example:
To set 3 horizontal workspaces…
gsettings set org.compiz.core:/org/compiz/profiles/unity/plugins/core/ hsize 3
To set 2 vertical workspaces…
gsettings set org.compiz.core:/org/compiz/profiles/unity/plugins/core/ vsize 2
If combine the two commands above, you will have a total of 6 workspaces (3 columns and 2 rows).
