Being able to scale each service (collection of containers from the same image) independently within a given stack (collection of services) is nice, because you may not always need to have a 1:1 mapping for each of those. As long as you internally use the magic rancher host names as indicated by alexR.
However, sometimes you may need to ensure that you have each service within the stack at the same scale. I had some odd requirement for this. The solution is to mark B, C, and D as “side kicks”.
My old thread about this is here: