It's worth keeping in mind that Nintendo is the only company that has ever managed to make games sell on portables. They've made 17 portable games that have passed 10 million so far (and 2, possibly 3, more released ones are bound to). Meanwhile, only 3 non-Nintendo portable games have managed to break half of that - Grand Theft Auto: Liberty City Stories (7.34 million), Monster Hunter: Freedom Unite (5.34 million) and Cooking Mama (5.32 million).
GTA: LCS released at the perfect timing. There was no other GTA releasing in its year, and the game was exclusive to the PsP. What made its sales, though, was its great legs, which was probably largely because there was no other GTA on a different system until GTA IV 3 years after it released.
A Call of Duty portable has none of those things going for it. It'll be competing against CoD on consoles, and its legs will be cut off by a sequel. Additionally, it's on a far, far less successful system than the PsP was in 2005, when it rivaled the DS for the biggest system of the year.
I'd say that 5 million lifetime seems incredibly unlikely, and that 2-3 million is more in the likely range. However, if it's not a good game, it could easily end up like the DS games and sell less than a million.
