Having said that, they can't ever seem to be able to plan how many pilots are needed.
That would be because the industry is a moving target with pilots resigning, retiring and losing medicals. Then the Dash is changed for an ATR, the pilot numbers change as more flying is introduced in one fleet and then flying is removed from another.
How the schedule is modified can make huge changes to the crewing requirements. Target more early business type flying means more crews or less flying during the middle of the day, reduce the early morning and evening flights requires less crew.