It's all up for the individual region to decide. The National solo rules have no say on the matter as National events don't use bump classes.
Usually bump classes use Pax.
Usually the goal is to get at least 3 cars to get a trophy class, but again, that's up to the region, some go with 2.
How you bump is also up to the region. In my region, in the situation you describe, all 5 cars (each in one car classes) would be thrown into the same bump class. 5 cars = 2 trophies, determined by pax.