Item Sets are groups of items that provide powerful extra bonuses when two or more matched pieces are equipped, up to a maximum of five pieces. It is possible to receive benefits from more than one set (e.g. getting the 2-item bonuses from four different sets).
If you Dual Wield or use One Hand and Shield, you have nine equipment slots open to you, for a maximum of seven total bonuses (e.g. the 2-, 3-, 4-, and 5-item bonuses from one set, and the 2-, 3-, and 4-item bonuses from another). If you are using a Two Handed or ranged weapon, you’ll only have eight slots, for a maximum of six total bonuses. Either way, you can receive only one of the unique 5-item bonuses.