In Ubuntu's Unity, you get four workspaces per screen, arranged in two rows with two columns each. Fortunately, there's a way to get more desktop space, but in Ubuntu 11.10 it takes a bit of searching.

After clicking around for 20 minutes, I finally found a menu in compiz' settings manager (package compizconfig-settings-manager). Select category "General", "General Options", tab "Desktop Size". There you can increase the horizontal and vertical size. You can also set the number of desktops, but I didn't notice any effect on this one.

I don't like to hunt basic settings deep down in obscure menus, so luckily, there's a quicker way for people who aren't afraid of editing the gconf database directly. You can manipulate the number of workspaces through the following gconf properties:

/apps/compiz-1/general/screen0/options/hsize
/apps/compiz-1/general/screen0/options/vsize

Use the command line utility gconftool-2 to assign new values:

$ gconftool-2 --type int --set /apps/compiz-1/general/screen0/options/hsize 3
$ gconftool-2 --type int --set /apps/compiz-1/general/screen0/options/vsize 4

The settings should have effect immediately, press Super_L+S (aka Windows key + S) to verify. As usual, you can switch to a particular desktop using Alt+<Num>.