Many item sets just require wearing 2 items from the set, some require 3 or more. Set bonuses are lost if the number of items worn falls below the minimum, and return when the correct number is met again, allowing a character to gear-swap in the middle of a quest and then swap back...
First, a question, just to be sure, when we talk about named items set, are we talking about a named set group like 'Ravenloft Item Sets', or are we talking about one particular set like 'Beacon of Magic Set (Heroic)'? I assume the latter. Secondly, how often do we foresee that ...