Limit number of files in a firebase storage path

I want to limit the number of files that a user can upload to a firebase storage path. Here is my firebase storage rule:

   service firebase.storage {
  match /b/jobsdbn.appspot.com/o {
    match /images/Business/{userId}/{imageId} {
       allow read
       allow write: if request.auth.uid == userId && (request.resource == null || ( (list(/images/Business/$(username)).size() <= 5) && imageId.size() < 50 && request.resource.size < 10 * 1024 * 1024 && request.resource.contentType.matches('image/.*')))

    }
  }
}

I have added list(/images/Business/$(username)).size() <= 5 from this article: https://groups.google.com/forum/#!topic/firebase-talk/ZtJPcEJr0Mc

but I always get 403 and not sure if "username" is global variable or not but I have used different variables and still cannot make it work.