If you are primarily cruising, a single car per side should be adequate as long as they are easily adjustable. Spring loaded pins are more convenient that threaded ones. I doubt you can justify the cost of under-load adjustable cars, and in any event they would be overkill.
I don't think you need to go the bearing car route either.
The only reason to carry 2 cars per side is if you tend to get overrides when using the forward-most position on the track because the sheet lead angle is too flat. (unlikely on a C.C boat) You can improve the angle by leading through the second block near the end of the track as well.