I have not seen anything on price or set makeup for number of cars in a set. I would imagine a basic set of 6 with one power as that fits in their book case sleeve, but that is pure speculation on my part.
I just got my 8 car set, and it is a really nice runner. Very smooth and quite and fast. Thought it would fly off the track at full speed. Can't wait to get the light kit for the cars. How a bullet train got on my SP rails is any ones guess but its still a lot of fun to play with and the full set for $350 is a great deal. As said it is one powered car with a TT. Will have to get a decoder in it to play with slower operations.