I say ~20 million at most. I expect it to be great even by Pokemon standards, but even matching Gold/Silver seems out of the question. Indeed, I have to wonder if Pokemon will ever hit the numbers of the first two gens again anytime soon.
Granted, I think most companies would be more than happy with 20 million units, but still... :p
Have some time to kill? Read my shitty games blog. http://www.pixlbit.com/blogs/586/gigantor21
:D