It's largely the conductor's call, a decision which they should be making based in part upon the manifest and the expected number of passenger. For some Regional trains especially they may well leave a coach or two closed to avoid confusion at stations with short platforms where all cars won't platform if they don't need the cars.

Finally, although I don't remember the actual numbers, there is a limit to how many coaches 1 conductor is allowed to work. Remember that a conductor has many other jobs besides collecting tickets; in fact that job is actually low down on the totem pole of importance. Now all trains have a minimum of 2 conductors, an assistant conductor and a the conductor. For all but the longest Regional trains that's enough staffing. I think if they get over 10 cars however, it would require a third. At which point if there are only 2 conductors, they still couldn't open more cars.