Halo is a 8-10 million per game franchise (of the #'d versions) so a lot of casual fans won't jump on till the real sequel is released.
I think that if you are expecting anything close to 10 million, that you're very optimistic.
But hey, it'll be interesting if that happens. It would take either a historically unprecedented attach rate, or an effective doubling of the install base for that to happen, right?