It's cool seeing an Elixir company have such explosive growth.
Elixir is part of their great performance! Their engineering blog is really interesting and full of neat information scaling Elixir (which is really hard to find since Elixir scales for a VERY long time).
We've been able to handle millions of concurrent HTTP(!) connections on a single machine for years; it feels like a pretty solved problem. Although, a lot of that involved userspace TCP stacks and really high-end networking hardware, so if you want to stay within saner territories you can scale that number back a bit.