As I understand your question.
One way is that you have to get specific equips with set affix. 1 permafrost weapon + 1 permafrost ring = (2)
Another way is that you need the Elixer mythstone on a accesory and ring to get +4 set affix(Cap). (+2 each)
Nadroji Crystal and Ring has a Legend affix of +2 set number
Look at my Hunter Ring And Nadroji Crystal it has +2 set number affix.
Even if I have (1) piece of the set affix it all becomes (5) because of the +4 set affix