Using the elixir mythstone on the Amulet and the Ring. The affix is 2+ all sets . If you have 2 of the 2+all sets affix from the Elixir Mythstone or the legend affix found on legends such as Nadroji Amulet with ring or the haunting amulet .
Since you have 2 of the 2+ All sets affix, this means you reach the total cap of 4+ All sets.
This means Crystalline(1) can turn into Crystalline (5) as 1+4= 5 when you use the set affix on one slot. If you have 2 slots allocated for the Crystalline set affix with the 4+all sets, Crystalline (6) happens as 2+4=6.
The cap of set affixes are (8). To easily get (8), you need +all sets affix. With 4+ All sets, if you use 4 slots for a set like plagued , instead of plagued (4), it will turn to plagued (8) as 4+4=8. (5) on most sets is usually more than enough to experience enough power and be efficient to use other set affixes so essentially, you can have up to 7 set affixes in use (8th is trophy but that's only active in Multiplayer).