We're considering moving our membership site from WP to statamic.

What are the scaleability limits for number of users? I'm not that familiar with flat file, what would happen if there were like 1m users? 2 million?

With DB there are always caching and various optimizations, and finally load balancers and whatnot as you get to enterprise scale.

What is required (if anything) - to get statamic capable of handling for example 1 million members, who don't have access to the control panel, but whom have access to some content/not other content etc, but who also interact with the site ( viewing/saving their own data).