We have established a McGuffin: rings require mithril, and all the currently available supply has been used up. So next season Sauron is going to have to go cap-in-hand to the dwarves to get more. How will he pay for it? With seven rings. Oh yeah, and a balrog. It's quite clever really.
I have an idea they have planned this seasonal structure for the show:
Sesson One: Three Rings
Sesson Two: Seven Rings
Sesson Three: Nine Rings
Sesson Four: One Ring
Sesson Five: The Land of Mordor
Also: LotR: good protagonist on a quest to destroy the Ring, RoP: evil protagonist on a quest to forge the Ring. It's just the sort of mirroring these writers seem to like. They are not going to make the forging of the One Ring easy for Sauron.