With the introduction of Fairy type, the total Types has increased to 18. This has increased number of possibly types (unique, i.e. Primary/Secondary is same as Secondary/Primary) to 171.
How many of such combinations have been used and which combinations are still left to be used i.e. No pokemon exist for such type?
Examples off the top of my head? Bug/Dragon, Electric/Poison etc. Unique Type used in Gen 6 - Water/Fire - Volcanion
Edit: Gen 7 Introduced. Thanks @Kenny980 for updating!