You know, when I worked at the hobby shop and could get a discount, I bought every billboard refer I could get my hands on from MT, Red Caboose, Intermountain, Life-Like, Bev-Bel, Atlas.....you get the point. I had over 400 refers in N scale and no place to run them. I could put about 100 on the layout at a time, but wow, was that a long train. Then I sold it all. :(
So today I have a larger scale and only a total of 15 cars.
To answer the question........I wouldn't add more cars. Just more layout.
B-)