how to set number of desktops

Ever since multiple desktops appeared, I've been used to having 4.
When I switched to Ubuntu (7.04), no problem - there was an item in the preferences
menu (sorry I can't remember what it was) that allowed me to set it.
Since then I've upgraded twice and am now on 8.04. The number of desktops migrated
with me.
I now want to set up account for my wife, but can't find the item to set it in preferences.
I'd hate to be stuck with only two, so can you tell me how to do it?