@Sup321V , @a63nt-5m1th

The only native function I could find that refers to more than 8 seats is this one. Having said that, I was never able to get more than player + 7. In theory you would have 1-14, 0, and -1, or 16 total (-2 does not count).

TASK::TASK_WARP_PED_INTO_VEHICLE
void TASK_WARP_PED_INTO_VEHICLE(Ped ped, Vehicle vehicle, int seat) // 0x9A7D091411C5F684 0x65D4A35D b323

Seat Numbers

Driver = -1
Any = -2
Left-Rear = 1
Right-Front = 0
Right-Rear = 2
Extra seats = 3-14(This may differ from vehicle type e.g. Firetruck Rear Stand, Ambulance Rear)