Theoreticaly a 14 x 12 should be big enough to contain all brood most of the time. Unfortunately there will be a honey arch at the top of each frame as the Q/E will tend to be a barrier for this over two frames.
Further, in most situations there will be no brood on the outer sides of at least the outside frames of the brood box. That lowers the number of available brooding cells further, but the beekeeper may have crammed in the full complement of twelve frames if a National with Hoffman frames.
Set against this, there will doubtless be areas of drone cells, too.
So several possible variables but two boxes of 14 x 12 will have around 140k possible worker cells.
Now think of how many eggs per day she is going to need to fill a double brood 14 x 12 for a turn-around of, say, a 25 day cycle to fully utilse those cells for brooding. It ain't gonna happen. No way Full stop. Yes, she may need a bit of extra space (beyond the single box) for a short time at peak lay-rate, but that is all, and only because a lot of cells are unavailable for brood.
Now, if the beekeeper does not make sure that the available space is utilised, there is the possibility of needing two boxes. But leaving loads of honey in large brood frames for later in the year will lead to possible extraction problems or not being able to recover as large a crop as possible, so not good practice, especially if a commercial enterprise.
YMMV, but that is my take on it.