Amazon Elasticsearch is scalable. The solution has scaled well for my use cases. However, I have not used it in a large installation. 

It is scalable enough for us. 

We cater to approximately a thousand clients, and each of these clients represents training organizations with a varying number of actual users, typically around four users per client since they pertain to trainers alongside their students. These clients vary in size, with some having as many as four hundred thousand students, while others have just around a hundred. We estimate that our platform serves a user base of at least twenty million individuals. The scalability of the system is a notable strength. It can be readily scaled, particularly when using it as a managed service.

Definitely, this solution is very scalable.

We are a very big organization. We have approximately 50,000 employees across the globe.

Most of them are using AWS services, so I'm sure they are also very versed in Elasticsearch.

If I had to give the scalability a number rating, I would give it a 9. I can scale with a single login, and I am able to get the server or update the server at any time.  Furthermore, I can consider multiple servers and can easily manage the load balancing using the multiple server on a  single login.