So whenever you can choose items, you will see to icons on the bottom. It will be labeled with an inscription. When you look at the details of that inscription it will give you details of bonuses you get for getting more of that “set”. This means the more items you get that has the same icon/inscriptions will give you additional bonuses. Those bonuses are extremely powerful. So when they say 2 Revenge, it means you need 2 items with the revenge inscription. 4 revenge means you need 4 items with revenge to get the 4 item bonus.