That stackoverflow infra blog post is out of date. They use more than a single webserver now. For example: https://stackexchange.com/performance
They still serve a lot more traffic than I do and I have hundreds of instances; thousands of containers.
Scale can depend on many things.
Here's a couple of reasons why it can easily be thousands:
1) Cronjobs, CI jobs, ETL, FaaS are all systems that exist. What used to be a process is now a container. (one need only check the PID count on their local machine to know that this can be many quite easily).
2) Microservices; I'm a larger fan of fat "services" but doing actual micro services tends to leave you with a lot of containers running
3) Actual compute need. If my original hosting strategy was thousands of machines, well, I'm going to have thousands of containers, if not more.
A similar “scale” e-commerce site would be significantly more load, have more dynamic data, and just be overall harder to run.
i don't remember who said it but a quote i really like is "it's not finished when there's nothing left to add, it's finished when there's nothing left to take away"