This is a pure numbers game. X a/c need Y pilots. Because of expansion plans they'll need Z instructors working in XX simulators over YY weeks to achieve the training plan to cover known resignations/retirements & new airframes arriving.
What is unknown is how many will leave in the coming 4 years. They can include an estimate in the calculation. Training new cadets is limited to certain times of day, e.g. no night shifts. As the cadets are being churned out of the sausage machine there need to be new captains. That requires TRI's (captains) in the later stages, the cases only SFI's. (F/O's). Simulators are full already.
RYR say they will hire 600 pilots by May next year. They are already in the pipeline. They seem to suggest these 600 will be extra. Impossible. I'm sure many SFI's are quitting teaching to complete their own command courses. That adds to the problem that more SFI's will need training up, and that takes TRI's. All in all these processes are not quick. Recruiting cadets and them being ready for line checking is >6 months. To recruit & train a new SFI to be released for solo teaching is >2 months.
This is not a 'mis-management of rostering department'. The expansion plans and recruitment plans come from higher up; the very top. You can have all the a/c you want, but you need pilots and for that you need trainers & for that you need simulators and for all that you need time. Time & resources are what top management is all about; and they can not create time. May, next summer season, is just around the corner.