Piecing together the scant info we have in the absence of any SE press release, it would seem Rebirth debuted around 2.5 million sales, the same as Dragon's Dogma 2. Of course, FF games don't have great legs while Capcom games generally do.
I wonder what that will mean for Part 3 since that should reasonably be expected to debut even lower than Rebirth. They'll 100% still finish the trilogy but some budget cuts might be in the cards if they don't get another exclusivity deal.