We do get projections from the airlines on how busy the day will be but they are often wrong. More than one occasion the briefing went "be prepared for a busy day" and it was slow. Or a day that was supposed to be normal we end up moving 8,000 passengers.
Last year they did put on a few more volunteer overtime people in case there were a lot of call offs. With the mandatory overtime rules in effect this year it probably won't matter.