Yes, there are. There are internationally accepted standards, and the bureau abides by them. What you see here in Parliament is typical. The team strength is based on the length of the event, so typically for your meetings, if they're two hours, we may send two interpreters. You're right, the interpreters will spell off in rotation. Interpreting is a cognitively demanding task. We would not expect an interpreter to do her or his best work non-stop for hours on end.
The interpreters also are there to work for their clients, so there are times when we.... These are principles and standards that we respect in theory. If a meeting goes over by 20 minutes we don't usually send in another team. That's a little context for you.