I think you're allowed two (laptop and desktop) unless you have Home and Student Edition, which allows three. But you lose Outlook if you pick H&S.
From WSJ's Walter Mossberg:
"In another move that will likely annoy many consumers, Microsoft has stripped Outlook from the low-price home version of Office. This version, which costs $150 and can be used on up to three computers, was formerly called Student and Teacher edition, and now has been renamed Home and Student. Formerly, you were supposed to have either a student or a teacher in your household to buy it, though stores never checked this. Now, that pretense has been dropped."
(http://ptech.wSportsJournalists.com/archive/ptech-20070104.html)
