Here's a blog post on scalability from the front page (well, sort of, see last paragraph): https://www.process-one.net/blog/ejabberd-massive-scalabilit... This includes not just textual claims about scalability but a lot of hooks you can use to followup on, like a reference to the Tsung benchmarking tool you can use. If you're asking about this for serious reasons, at this scale you're obviously running your own tests anyhow. You may also want to speak directly to Process One about this because it sounds like you're at a scale where you should probably be looking at paid support anyhow.
I'm not necessarily endorsing this, just giving it to you as some first stabs at answers and some ways to follow up.
If there's anyone reading this from the ejabberd project, note that the link to this on your front page under "Massively Scalable" is completely broken; that links to "blog.process-one.net" but that domain is completely dead, so it doesn't redirect to the link I gave above. (It's also part of why I posted this; my post here is not a "just read these docs" because I had to do non-trivial work to find them.) I had to pull that out of archive.org. Should probably check for any other links to that domain too.
I'm past my edit window now so I can't remove the claim, but I will verify and agree that it is fixed now. If you (the reader, not Metalhearf) previously visited the home page you may have to manually reload to bust the cache to see the correct link(s) now.
I'm not necessarily endorsing this, just giving it to you as some first stabs at answers and some ways to follow up.
If there's anyone reading this from the ejabberd project, note that the link to this on your front page under "Massively Scalable" is completely broken; that links to "blog.process-one.net" but that domain is completely dead, so it doesn't redirect to the link I gave above. (It's also part of why I posted this; my post here is not a "just read these docs" because I had to do non-trivial work to find them.) I had to pull that out of archive.org. Should probably check for any other links to that domain too.