On intercontinental flights 3 pilots are standard.
@Astro14 can shed some light here.
There are three FAR (Federal Aviation Regulations) under which an airplane can be operated.
Part 91 - General.
Part 121 - What we would know as Major Airlines
Part 135 - Charter and smaller operations
In parts 121 and 135 there are limits to the duty day (how long a pilot is on duty) and the flight time (block to block is flight time).
Harrison Ford’s Gulfstream is likely operated under Part 91, because he owns it, and he isn’t a regularly scheduled operator, and so, those pilots can be on duty, and at the controls, for as long as Mr. Ford wants.
But for 121 and 135 operators, Pilots are limited to 14 hours on duty and 8 hours of flight time. Scheduled. Flight beyond that requires a 3rd pilot. Flight beyond 12 hours requires 4 pilots. Beyond 16 hours (and yes, there are some) requires 5.
So, EWR -ZRH is about 8 hours block time. But ZRH-EWR is closer to 9 hours. So, a 121 or 135 operator is going to have 3 pilots on that flight, to cover the return flight. Doesn’t matter if it’s a Gulfstream or a 767, flights over 8 hours need to have 3 pilots.
Heaven help the operator who plays games with that rule. Cut it close, and get delayed taxiing out, you just became illegal. A major Airline scheduled London - ATL years ago as 7:58, based on zero taxi delays and an unrealistic cruise speed (VMo) So, they crewed it at 2 pilots, because it was “under 8 hours”.
Many pilots would just taxi back to the gate if they encountered delays, canceling the flight. The flight was chronically delayed, with the specious planning assumptions that went into that 7:58 flight time.
The pilot’s union sued. The FAA weighed in, the airline was fined, and that flight has 3 pilots now…