In the original release of D2:ED there were only 4 sets. Three 10 piece sets were each spread out in a separate flying fortress: Stone's fortress contained the Archmage set, Raze the Hunter's set (ranger) and Kali the Scorpion set (warrior). Mindreading those 3 bosses as well as Xanlosch (in Stone's fortress) will give you the four piece Ulthring's set.
In DKS, there were additional sets added to the game, and the locations of the previous 10 piece sets were changed.