I believe Joe had showed in another post that there's a 500 limit somewhere. but he also showed how you can override that. Nevertheless, I'd never approach 500 sites on one server, let alone one CMS. I'd think 200-300 smaller sites on a single web server would be enough. I know most hosting companies put a lot more sites than that on a server (like a couple thousand), but not for me personally. After that amount of sites I think you owe it to clients to get another box.
We've got a dedicated web server with maybe 75 stand-alone sites along with a Mojo install and 20 sites on that. We also run email off the server and SQL 2005! It's only a P4 3.Ghz CPU, 4GB Ram..... and the server only runs between 5 and 30% utilization. You'd be surprised what a properly configured Windows server can do. We're looking to upgrade though, just because.
Stress testing would be interesting. My friend Matt has a nice test server, maybe we can get him to load up Mojo and look into ways to synthetically stress test it.
**At my last job I was in charge of a DotNetNuke CMS for the company intranet. I had a single install with 51,000 pages, 300 child portals and an unbelievable amount of users, documents, videos, etc. Never had any performance problems. And mind you that I really don't like DNN at all, loath is more like it honestly. Mojo is tightly built, modern in structure, not bloated and just feels snappy.