My math pretty much works out as follows.
First, the low end. It would be pretty shocking if they couldn't ship 1 million in Japan + Asia. They usually do about 2/3rds of their business outside of Japan, but Japan is depressed due to the PS4, so I'll round up to about 1.25 million per NA and Europe each. Put in 500K over several years for an eventual PC port (FF13 has pretty much done that) for 4 million.
On the higher end, let's say it hits 1.5 million in Japan, sells 2/3rds outside of Japan for 1.5 million in each other region for 4.5 million. If we put in 750K for PC over two years (which is more on par with what Metal Gear has done in ~4 months, and FF7 has already done 920K on Steam), you get 5.25 million. If the game is an ~85-88 type of title, on the console side maybe you get something like 2 million in North America and 1.75 million in Europe instead of an average of 1.5 million each and hit 6 million.
Now, if they just totally blow it out of the water, sure, it could get a lot higher, but I'm not writing that down into the expectations until I see something that really screams "Wow this is going to be huge!" at me.