Arithmetic is quite an exact procedure. There is not much new about training. XX a/c, short haul, require Y crews per a/c. That's easy. Y crews require Z trainers mixed with SFI/TRI/TRE. To achieve that for Y crews you need A simulators. If you order B a/c to be delivered over the next 5 years it will create an expansion of B a/c to bring ther total to XX + B - any sales. Each year a % of B a/c will arrive. In advance you need to train a known number of crews. Therefore in advance you need an increase in trainers & simulators. Inherent in those calculations you'll need to factor in a captain upgrade program. Included will also be the number of retirees (known number) and those leaving, often 10% loss.
It's been that way for decades. I was associated with an airline who had a very aggressive order book, but did not increase their training dept' and sim availability at the same time. They lagged behind. The left & right hand did not seem to be connected; either that or delaying the increase in trainers and upgrading captains was seen as a cost saving until the last minute. But then you start cancelling flights, or you try & delay deliveries. Either way the forward planning bloke/committee was not up to it. The overall oversight rests with CEO.