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Under normal 2 pilot part 121 ops I could only be scheduled to fly 8 hours in a given day. This is plenty for the routes we flew (longest leg around 4 hours) so I never encountered the rules for longer flights.
For flying beyond 8 hours -- long haul international ops -- what determines how many pilots must be on board? Is only flight time taken into account or is the number of timezones crossed also a factor?
The regulations for this are in 14 CFR 121 Subpart R—FLIGHT TIME LIMITATIONS: FLAG OPERATIONS.
In short, flight time limits within a rolling 24 hour period depend on the number of crew members assigned to the flight:
However, they may also implement a "Fatigue Risk Management System" which allows them to exceed these limits if they are found to be safe and approved by the FAA. This system would most likely address the number of timezones crossed but is optional.
A particular airlines pilot contract may also have more restrictive limitations (like considering the number of timezones crossed) that are negotiated by the union, but this is not required by the regulations.
§121.481 Flight time limitations: One or two pilot crews.
(a) A certificate holder conducting flag operations may schedule a pilot to fly in an airplane that has a crew of one or two pilots for eight hours or less during any 24 consecutive hours without a rest period during these eight hours.
...
§121.483 Flight time limitations: Two pilots and one additional flight crewmember.
(a) No certificate holder conducting flag operations may schedule a pilot to fly, in an airplane that has a crew of two pilots and at least one additional flight crewmember, for a total of more than 12 hours during any 24 consecutive hours.
...
§121.485 Flight time limitations: Three or more pilots and an additional flight crewmember.
(a) Each certificate holder conducting flag operations shall schedule its flight hours to provide adequate rest periods on the ground for each pilot who is away from his base and who is a pilot on an airplane that has a crew of three or more pilots and an additional flight crewmember. It shall also provide adequate sleeping quarters on the airplane whenever a pilot is scheduled to fly more than 12 hours during any 24 consecutive hours.
...
§121.495 Fatigue risk management system.
(a) No certificate holder may exceed any provision of this subpart unless approved by the FAA under a Fatigue Risk Management System.
(b) The Fatigue Risk Management System must include:
(1) A fatigue risk management policy.
(2) An education and awareness training program.
(3) A fatigue reporting system.
(4) A system for monitoring flightcrew fatigue.
(5) An incident reporting process.
(6) A performance evaluation.
