I love Rails, but those guys are too anxious to throw the Apache baby out with the PHP bathwater.
At the same time, it's trivial to switch this number if you've not hit your server resource limits yet. (i.e. when coming under a digg load)
Once server limits are reached (in Apache or nginx/mongrel/rails), the point is moot - you'll have to instantiate new servers at your co-lo / hosting company by hand anyway.
All that said, nginx is a fantastic web & proxy server. It blows Apache out of the water in terms of memory usage, etc.
My point is that the application processes are where things are tricky. Even 'big slow apache' can saturate fast connections with static content. The real trick is to get the most out of your dynamic code without under utilizing your resources (by launching only one mongrel instance, for example), or running things into the ground by letting your apps try and grab too much memory.