If you are looking for a Cloud operating system that allows you to scale to many thousands of nodes then OpenStack is your best choice.
Telcos, banks and research institutions are some of the most likely use cases for OpenStack but it is also possible to deliver well tuned small clusters for entry level uses.
OpenStack aggregates a large number of components specialised and optimised in performing specific tasks.
This modular structure allows you to integrate what you need and when you need it without disrupting standard operations.
The CERN in Geneva is probably one of the longest running and renowned examples of OpenStack’s scalability.
In 10 years CERN went from 0 to 300.000 CPU cores and hundreds of petabyte of data managed with OpenStack.
That is not the largest OpenStack installation in the world but it’s a good example of what you can achieve with it.