There will be multiple buses.
If you are arriving on embarkation day, you provide the flight information and Princess will know about what time you will be there to board the next available bus.
When I have arrived before embarkation day, I just pick a flight & arrival time to input that matches when I would like to leave for the ship and bears no relation to when I really arrived.
If you use EZAir for a flight that arrives before embarkation day, then Princess knows when you arrived and I just show up at the airport about the time I would like to leave for the ship.
Of course pick times that are practical, not too early as buses likely do not start before 9 AM and not too late as the last bus will be a couple of hours before check-in ends.