There was a YT preso from several years back where the StackExchange founder explained how it ran off just ~10 servers, and could run on half that many if neede

There was a YT preso from several years back where the StackExchange founder explained how it ran off just ~10 servers, and could run on half that many if needed. He stressed the simplicity of their arch, and that their problem space was massively cachable, so the servers just had a few hundred GB of ram, and only had to do work to rerender pages, but could store them in cache most of the time. It was a C#.Net app. So, I think there is a lot more in common than you think between HN and SO.