[pjb]

Sum the number of projects and folders and add one for the inbox, then multiply by 140616 and that's about as many possible unique perspectives available.

I have 21 in my perspectives window plus the 5 default, but in practice use only a few and no more than can fit on the toolbar for easy access. I anticipate using many more when tags arrive. I could see a more feature rich perspective browser being helpful (thinking iTunes browser, for example). Even so, I do not foresee using more than two or three perspectives in any single day.