I am traveling to Oklahoma state in USA next week. I am driving my SUV but I have 4 friends going with me and one more person will join us in Oklahoma. I am wondering if there is any legal limitation on the number of passengers allowed in one car in Oklahoma. Not including the driver seat, my car has 4 seats but there is some space in the back of the SUV so someone could sit on that. I can't find anywhere that tells the law on number of passengers allowed in one car in Oklahoma.
The only likely legal limitation is that there must be a seat belt for every passenger.
Some states only require seat belts for passengers in the front seat, or under a certain age (such as 13).
But to be safe, make sure every passenger is buckled in, and you will have no problems anywhere in the U.S.
As per the NHTSA summary, the number of occupants in the rear seat cannot exceed the number of available safety belts. Including the citations of the relevant laws on pages 19, 29, 33, etc.
This would imply that you can legally pile four people into the backseat and/or the cargo area. The only other laws I've been able to find concerning passengers and where they can ride either apply to larger vehicles (with a seating of 15 or more) or concern whether passengers can ride outside the passenger compartment (short answer: in the bed of a pickup truck or in a parade, otherwise no.)
If you are passing through any other states on the road-trip, the Governors' Highway Safety Association has a good collection of summaries of each state's traffic laws, including seat belt laws.