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Docker service update --replicas increase won't increase my container number

Good day.

I apologize in advance if this is an ultra noob question, but my OCD won’t leave me alone if I don’t get an answer.

I’m studying Docker Swarm in a course and I’m seeing the instructor doing some docker service update --replicas=X web (web = an nginx service) or docker service scale web=X for that same matter and, when checking with docker ps you can see that X represented in X number of containers.

However in my end (Play with Docker website) almost everything works fine. If I use docker service update --replicas=4 web it works fine. Even if I do a docker service ls you can see my replicas updated to 4. However when I do docker ps I can’t see any container and, according course instructor, it should appear 4 containers, right?

What am I missing here?

Thanks in advance.

Warm regards.