Useit.Com: Card Sorting: How Many Users to Test.
Fidelity Investments has one of the world's best usability teams, led
by Dr. Thomas S. Tullis, senior VP of human interface design. Tullis
and co-author Larry Wood recently reported the results of a study
measuring the trade-off curve for testing various numbers of users in a
card sorting exercise. [Tomalak's Realm]