Unfortunately, the answer is: it depends.
A basic Phoenix app that doesn’t hit the database and return a static string could probably handle tens of thousands of concurrent connections. While an app that does a lot of writes or big operations may only be able to handle 5.
You’re not wrong. The limits are very low for most use cases. You should be able to handle at least a few hundreds concurrency connections. You can play with the concurrency settings and see how it affects performance and load balancing.
You can also try type = "requests" under the services.concurrency block in your config. This sets the concurrency limit on the number of concurrent requests being handled (if your service uses the http handler, which it seems to do). This will likely become the default at some point. It allows us to reuse connections and put less stress on your connection accept loop.