Run your owntests. It’s the only way you’re going to get good results. Can be as low as 3, can be as high as 100+. Use httperf and test, test, test.
I’ve posted some test strategies on this form a few times… maybe do a search for httperf and you might see something I wrote a few weeks ago.
I’d recommend running more than one Mongrel unless you can delegate most of your requests to a static server using caching. Browsers need to grab your css files and js files, and if you’re not offloading that to Apache then one instance is really going to choke.