Do I need an orchestration tool to autoscale/scale on demand my web service?
Yes. And there are tools out there covering this piece of functionality. 

Probably the most popular one is kubernetes (see also kubernetes):


  Kubernetes (k8s) is an open-source system for automating
  deployment, scaling, and management of containerized applications.


Depending on the provider for your deployment environment the actual orchestration tool/service may be already included and all you'd provide would be your desired scalability configuration. For example when deploying on the Google App Engine Flexible environment the service scaling settings could look like this:


automatic_scaling:
  min_num_instances: 1
  max_num_instances: 15
  cool_down_period_sec: 180
  cpu_utilization:
    target_utilization: 0.6