For get a better idea of what Pokémon trend is:
Pokémon Gen 1 = 31.38 million
Pokémon Gen 2 = 23.10 million
Pokémon Gen 3 = 16.22 million
Pokémon Gen 4 = 17.63 million
Pokémon Gen 5 = 15.60 million
Pokémon Gen 6 = 14.70 million [STILL SELLING]
When all is said and done, Pokémon X and Y is gonna sell over 16 million, but under 17 million. (it sold 240k last quarter, up )
Pokémon XY YOY sales:
2013 = 11.61 million
2014 = 2.09 million
2015 = 0.76 million
2016 = 0.24 million <----- only 3 months (up YOY by 60% compared to the 150k of 2015)
This indeed show that the game isn't as strong as the launch by end of the 1990 years, but since gen 3 the game is pretty stable at 15-18 million.
Remake, as well, always sells a lot.
Pokémon Gen 1 remake = 12.00 million
Pokémon Gen 2 remake = 12.72 million
Pokémon Gen 3 remake = 11.84 million [STILL SELLING]
Pokémon Alpha and Omega will outsell the remakes of Gen 1 and Gen 2 with no problems too.
Pokémon ORAS YOY sales:
2014 = 9.35 million
2015 = 2.11 million
2016 = 0.38 million <--- only 3 months (down YOY by 35% compared to the 590k of 2015)
Now, we definitively talking about one of the bigger series ever, if even remakes can sells so easy over 10 million, and hell, even over 12 million.
Pokémon is selling just a bit worse than what it was usually selling on DS, that's even if we considering that when all is said and done DS will outsell 3DS by 2:1, in others worlds, the attach rate on 3DS is WAY bigger.
This isn't happen with others series, Mario is the best example. And there are even series which did way better on 3DS than on DS (Animal Crossing, Fire Emblem), but that's another story.
What really mean this is that Pokémon as series is still very, very big.
Now, 3DS is not in his gold years, indeed is not selling as good as first years, and we can say that in about 2 years should be death.
This is happen with Pokémon Black and White years ago, and indeed Gen 5 sold worse than Gen 4.
At first, i though this was gonna happen again, for the same reason Pokémon Black/White sold worse than Diamond/Pearl.
Though,we recently saw Pokémon GO... and holy shit. I don't think i should talk much about this, cause we all know the insane success it is. I have no doubt this will boost Pokémon Moon and Sun.
By an huge or small margin? We will see. Anyway, if at first i though Pokémon Moon and Sun was gonna sell worse than X / Y (like said, for the same reasons Black/White sold less than Diamond/Perl), now i expect about the same numbers, or as said, a very little difference...
In conclusion, 20 million will be possible only if Pokémon GO will be super influencing.