Around 16-17 mil. Should be big since it's a large step forward, but the recent glut of Pokemon releases may eat into sales a bit,

One thing that's relatively important, is that Pokemon games are incredibly front loaded comapred to other Nintendo games. Take Diamond/Pearl, for instance. It's the best comparison to X and Y, since it's the first on the DS.

In Japan, D/P sold a total of 6 million. 4.4 million of those were in its first 3 months (2006).  In the USA, 3.6 out of its 6.2 million sales were in its first 7 months (2007), and in Europe, 2.7 million out of its 4.3 million were. You also have a bit from outside of these areas, so say 300k from there up to the end of its first holiday season.

This means that roughly 11 out of Diamond and Pearl's 18 million were sold in its first year.  X and Y has a shorter first year (3 months in all territories, which is the same as D/Ps Japan, but less than the 7 months D/P had in Americas and Europe). As such, you can estimate that slightly more than half of X and Ys sales are going to be in 2013.

As they're so front-loaded, the size of the installed base is very relevant. In Japan, you're looking at a 3DS installed base of 13-14 million by October 2013 (assuming it sells as fast as this year, or - more likely - slightly faster), compared to the DS' 11 million when D/P launched. But in Americas, it's likely to be slightly smaller, with 10-10.5 million against the DS' 10.8, and in Europe, it's going to be significantly smaller, with roughly 9 millon to the DS' 11.8.

Another thing to note, is that in Europe, Pokemon has started selling less and less. In the GB days, over 25% of Pokemon's sales were in Europe, then in the GBA days, around 25, with Diamond/Pearl is dropped under 25%, then through the DS era it dropped under 20%, and then with Black & White 2, it's now at just 17% (although that'll most likely rise to 18-19% through 2013). You can see the drop in Europe pretty clearly in hard-numbers, too.

This leads to an overall assumption that in Europe,  Y & X will sell less than Diamond/Pearl. In Japan, it'll almost definitely sell more, as Pokemon has seemingly increased in population during the DS era (Black & White nearly matched Diamond/Pearl, which is incredible for a second Pokemon on the same machine). Add that up with the installed bases, and it's going to be significant. In America, Y and X should sell slightly less, because it won't get the same holiday boost as D/P did, due to the 3DS being unlikely to match the DS' simply insane 2007 holiday season.

In 2013, I'm thinking 5.5 million Japan, 3 million Americas and 1.8 million in Europe, leading to 10.3 million overall. It should go on to sell just shy of 7.5 million in Japan (meaning it ends at Gold/Silver levels), slightly under 6 million in Americas, and 3.5 million in Europe. Then probably around the million for "rest of the world", leading to 18 million sold. The increase in Japan offsets the drop in Europe.

About what Diamond and Pearl did I guess.

The only reason I put 25m+ is because of how different this one is, and the hype level that is resulting can result in some better-than-average results. The graphics have a huge upgrade from the usual, and there are a lot of changes that seem to be things a lot of people like. There seem to be people who haven't played pokemon in a while who seem interested in this one, so it isn't entirely impossible for this one to have better legs than most of the others.

You're correct that there's a lot of potential here. If there's ever a title that has the potential to sell over 30 million, it's this one. If you look at all the people who have ever bought a Pokemon game, it's probably passed 50 million by now. But, as the series hasn't evolved a ton, there's been a large drop-off. If you can get a large number of those drop-offs interested, you could very well end up selling 30 million. Personally I think it's unlikely, but it's definitely a possibility.

F0X said:

Around 16-17 mil. Should be big since it's a large step forward, but the recent glut of Pokemon releases may eat into sales a bit.

(I can't get quotes to work anymore, so I'm just copy/pasting your post

It's not strctly speaking true that there's been an increase in mainline Pokemon releases, though.  Look at it as a timeline, and it's rather evident.

1996: Blue/Red
97:  Nothing
98: Yellow
99: Gold/Silver
00:  Crystal
01:  Nothing
02:  Ruby/Sapphire
03: Nothing
04: Fire Red/Leaf Green, as well as Emerald (so 2 major Pokemon games in one year)
05: Nothing
06: Diamond/Pearl
07: Nothing
08: Platinum
09: Heart Gold/Soul Silver
10: Black/White
11: Nothing
12: Black/White 2
13: X and Y

Pokemon games have always been a close to annual thing. In the GB era, there were 4 games in 5 years. In the GBA era, we saw 3 games in 4 years. On the DS, we saw 5 games in 7 years.

It's abnormal for Pokemon to start a new generation the year following the end of the previous one. Last time that happened was the transition from Gen 1 to Gen 2, when Gold/Silver followed the year after the release of Pokemon Yellow. Just by looking at the pattern, I'd have anticipated a Gen 3 remake instead on a new generation. However, I think Game Freak is making the right choice by using a new generation for the 3DS transition, and doing it quickly. Though I do think X/Y will be competing with B/W2 until after the holiday season is over.

I'm not sure how they'll be competing, really. Black & White 2 is going to be in the sub-30,000 a week levels by the time X/Y launches. It's likely it won't even do 500,000 post Y/X launch, really. Meanwhile, Y/X is going to be selling somewhere around 10 million. That's actually less of a competition than Mario Kart DS is to Mario Kart 7. 

18-20
my vote

Mario Kart follows a completely different model. Going by the history of Pokemon sales, I find it very unlikely for B/W2 to sell so little in its second year on the market. I haven't seen nor heard of any main Pokemon game making such a huge drop in sales. Granted, I've hardly heard of a new Pokemon generation starting so soon after the end of the previous one.

3 years?  Black and White 2 are part of Gen V.