I think the real question is less about how many units are available to play the game as much as its more about whether FF is still relevant to today's market. As Bolivar kinda pointed out, RPGs don't really seem to be drawing the same level of fandom on next-gen consoles as they did in previous console generations.

I don't think it will sell very well unless SE really tries to pull out a VII style PR campaign (which signs are showing they really are not trying) to get the numbers they need. I still feel that FF no longer has the pull it did ten years ago, while still a milestone series I don't think it will make the numbers VII did unless SE can convince the consumer market that XIII will be another Gaming Revolution and something you don't want to miss much like VII did.