Ramrise
Commanders have discretion (up to 3 hours) for use to cover things such as emergencies,delays etc. On a single sector there is normally some "fat" in the schedule between the crews rostered FDP (flying duty period) which consists of a period time to report for the flight and the scheduled flight time.
In the case of such a long flight as this topic crews can report and run out of hours within 15 mins with a small delay (i.e. late pax). If this happens crews can use their discretion or otherwise. You may ask how can it be that a Crew member turns up fit to fly then a few hours later he's not. Difficult one - the landing is some xx hours away. Clearly most posters think 2 crews is the answer, best option for time away from controls. Only 3 crew means less time away. I'm probably safe in saying most (state)airlines use 4 crew