Well, damn, I was right the first time. Having 5 users requires no CAL purchase. Having 6 users requires purchase of 6 CALs. Having 10 users requires purchase of 10 CALs. That's a huge penalty to go from 5 to 6 users.
You get 5 users with TFS and the 6th user requires an additional CAL. So you only have to purchase 1 CAL for the 6th user.