Yes, around 65k connections before you hit the ephemeral port limit. I build websocket and TCP proxy services, so tens of thousands is normal, hundreds of thousands, also not a big deal. That type of concurrency is no problem for the Beam on dedicated hardware with a few CPU cores and some memory and a rather appealing use-case for Elixir/Erlang in general. There’s been some nice blog posts on millions of connections with Phoenix.
Caddy (and Traefik) can consume a lot of CPU/memory when handling 30-50k persistent connections, on a smaller Digital Ocean VPS Traefik was using half the machine resources to do it. We had to remove it and have the service directly on the host with no reverse proxy in the path due to its resource usage and concurrent connection limitation.
My company is currently setting up our own version of a deployment system for our Elixir apps, so when I see projects like this I always hope that they take an approach that at least allows for the full capability of the Beam to shine without restrictions that reduce its utility for my use-case so maybe we won’t need our own custom system. 
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