I usually play it by ear, and figure that each "seat" is for a geared runner, and not just a typical adult. Sure, a minivan might fit a family of 7 comfortably.. or uncomfortably, but a Runner Team? Lucky to get 3 + the driver in one of them. I drive a minibus, and sure it has 20 seats (besides the driver), but I couldn't fit 20 Runners in there.
If you look at the Ares Roadmaster picture in SR4's Used Car Lot, and the seating guide in SR4a p.348, that thing can seat 10-14, yet in SR5 only 8.
So I play it by ear, look at the Seats, look at the Body of the vehicle, and just go with what makes sense. Figure a Troll takes up 2 "seats" but in a pinch, you can also throw someone extra in for a tight fit. Look at most of the motorcycles. 1 Seat, but you could probably fit a passenger on most of those too.