Try as I might, not only can I not find an answer to my question, but it seems I'm the only one to ask it:

I can't find a way to change the number of virtual desktops per activity that doesn't affect all activities. For example, activity A has four desktops and I'd like activity B to have just 2 desktops. Is this possible?

I'm currently running KDE 4.7.4

If it is not possible, this is a major flaw in the design of activities. Surely, if the purpose of activities is to improve workflow across different projects, there has to be a way to change the number of desktops used for any given project without affecting all the others.