It won't become the best selling Pokemon game ever. I can tell you that with complete, 100%, confidence.

The most I could see it doing is 20 Million. But that is on the utmost extreme side of my prediction. 16-18 Million or so seems more of a realistic number. (With the possibility of overtaking D/P. Maybe.)

there is so much hype for this game it might sell over the 20m, it better not be dissapointing!

I don't think this game will sell much better than Ruby and Saphire. 3DS's sales will limits it sales and I don't expect this game to sell 3DS like how Brain Age, Nintendogs, and New Super Mario Bros. sold DS. It may have a shot in Japan and be the Final Fantasy 7 of Pokemon RPGs.

They are prepping and advertising this game a LOT. I would say it will sell over 20m because I know a lot of people who are excited about the changes and there definitely is a lot there to bring in new comers, bring back old ones, and keep the ones already there.

That's really shooting it low.  The 3DS sales are really high and the current install base for the 3DS really shouldn't be something to hold back sales of a game like pokemon that drives up software sales.

Off-topic, I remember someone saying something kind of dumb.  I don't know if anyone here would agree with it, but he thinks that 1st party games are only made to sell hardware.  He thinks that no 1st party dev makes a game to make money off of the software and only do it for hardware profit.  I think he has no idea what he's talking about, and i told him exactly why he was wrong but he still thinks he's right.  
Anyway, only reason I thought of that was because I was talking about Pokemon X and Pokemon Y when it was brought up.

I think the sales will be huge, though I'm not sure on the exact number.

I think 1st party games are made to sell HW while still making money off of SW.

Although, isn't every game supposed to do just this? How could you make an argument out of that?

Well, I do not doubt that there are games (if not most 1st party games) made to sell hardware.  However, if you look at some games that 1st parties make you can see that they obviously had no significant effect on HW sales.  I mean, Fire Emblem: Awakening barely sold any 3DS HW.  It's SW sales is really where it did anything worth a damn (plus having lots of DLC).

I have a feeling that Nintendo plans to market this game more than any other Nintendo game of the last decade. They will really attempt to push it...hard!

It also seems to be the first completely fresh Pokemon since Pokemon 2. I skipped Black & White but do plan to buy this one by the end of the year.

I predict 20 Million+ with it having a chance to outsell Gold / Silver.

this is a HUGGGGGE stretch but imagine if it outsold Gold/Silver! Thats Some Major Accomplishment. Pokemon Z should be coming out 2015 holiday and hopefully that sells well too