Hi,
Have you explored creating custom scaling rule based on metric and then selecting storage account -- files -- file count, etc., with the condition(s) and options for increasing/decreasing instances? I know the metric for file count is an option, however, I've never used it as a metric to scale VMSS.
Under what specific circumstances/conditions do you want to scale out/in? If you explain in more detail it would be helpful to me to recommend a more specific approach. Otherwise what I can recommend is experimenting with the scaling in the portal with the metric(s) you are considering and then based on that ask additional questions in a comment.
-TP