Hi Eyal, what a beautiful name 
You can look at the APM server as a scalable ingestion layer for data produced by APM agents into Elasticsearch, so certainly no need for a container per agent, unless very specific restrictions/limitations enforce that.
Regarding sizing, see the relevant documentation and look for former discussions like this one.
However, the actual required size largely depends on each specific use case. I'd say the easiest way to onboard would be a cloud free trial. Just spin off a cluster with an APM server, start sending APM data into it, and see how your apm-* indices behave over time. Of course you can do the same with an installed APM server.
Since you are expecting mostly Javascript and Java, I would also suggest you follow through this useful blog post that includes a sample app.
I hope you find our APM tool useful and easy to use.
Eyal.