They where dynamic publishing sites, so not supercomplicated and absolutely cached to the brim. A big chung of the concurrent visitors (prob like 90%) where on two websites, so two installs.

Ok. Yeah, you could easily have 100k concurrent visitors with that, if the request isn't actually reaching php/WordPress. On e it does, you're down to something like 2-4, maybe 10, requests per cpu core, per second. Depends on the hardware and how the site is configured.