I got a cockpit tour of a United 767-300ER in year 2000 that was used for
overseas international flights. Crew said they have 3 INS units on board.

my friend just got bumped down off the 767 with UA. I remember him
saying it had 3 INS units. He used to fly the 777 and said, "we
weren't even told how many INS it has" apparently due to the systems
on the 777 having so many redundencies (this is from memory). I think
he also said that the 777 has 9 hydraulic systems. If 8 of them
fail, the remaining system can power the other 8.

Gerald