One interesting aspect is that the number of servers is much higher than what would actually be needed to run the site, most servers run at something like 10% CPU or lower. Most of the duplication is for redundancy. As far as I remember they could run SO and the entire network on two web servers and one DB server (and I assume 1 each of the other ones as well).

If someone says SO runs on a couple servers this might be about the number actually necessary to run it with full traffic, not the number of servers they use in production. This is a more useful comparison if the question is only about performance, but not that useful if you're comparing operating the entire thing.