In a nod to massive (and it must be said, justified) optimisim, Square Enix CEO and President Yoichi Wada has stated the compay's sales goal for Final Fantasy XIII to be 6 million sales. Considering that, well, it IS Final Fantasy and besides that they also have the impressive Dragon's Quest series as a pedigree.
I'd say that they should have no trouble hitting this mark, unless of course the fumble the game mechanics. However, though some Final Fantasy games have been more popular than others (FFX is still most popular), it is a sure bet that anything Final Fantasy will sell in Japan and here, for those of us that like a deep, compelling story.
Final Fantasy XIII will be released to Japanese PS3’s in December, with a western version shipping in spring 2010. So not too much longer to wait.