This was bugging me for a while until I came across the answer. It says in the manual and all the simbrief profiles that the challenger 650 carries 12 Passengers. But there only 11 seats, unless you count the loo. But who will want to sit in the loo during the flight. I didn't want to seem stupid by asking on here, so I just excepted it was probably a configuration thing, and our challenger was configured for 11 passengers. I even edited my simbrief profile for 11 passengers.
Then I watched a YouTube video of someone showing around the cabin of a real world CL650. And in the galley they opened a cupboard and pulled out a folding seat. When in use it can face forward just behind and in the middle of the pilots acting as a jump seat. Or it can face the rear of the aircraft to accommodate a stuard/stuardess. I don't think a paying passenger would be expected to sit in this seat, but it would provide seating for 11 passengers and a stuardess. If anyone is really interested to see the video I could see if I can find it and put a link in this thread
We learn something everyday. It's just a small point but it made me happy and it cleared up a mystery.