I've been running https://nextchessmove.com/ for 12 years now. I've learned a lot while scaling it. It's currently 150+ servers according to a quick grep | wc of my ansible inventory: mostly VPS instances, but lots of dedicated servers. It's gone through several (hopefully unnoticeable) major rewrites to keep complexity manageable: CoffeeScript+jQuery to TypeScript/React, Ruby/Rails to Elixir/Phoenix, etc.