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.