"Just" over 12 million units?
Look at the first weekend iPhone sales stats. (Apple didn't release the numbers for the 7)
For the 2017 iPhones I would have expected something like 15M for the first weekend, so 12M for the X and a few million iPhone 8 sales would probably add up to a number close to that.
Of course the demand for the X may well exceed 12M at launch, but I don't see it as a catastrophic supply constraint that some seem to imply since the numbers seem to be in line with previous years. The question will be more about how fast they can replenish their stock supply after the launch.