Disclaimer: while I did not work on those scale tests, I am one of the maintainers of one of the open source projects used in those scale tests.

While the largest fleet I have observed under a single hub managed was fewer than 500 clusters, reviewing those performance results provided good insights.

Thought of sharing here as well since on Linkedin a post by the author on those scale tests went viral (almost 700 likes and 50 plus comments)