In the context of Rails applications hosted on EC2, then I've not found Ruby's memory usage to really be an issue. In my experience most Rails apps range between 150-500MB per instance.

My current employer typically uses M5 instances which have a ratio of 1 vCPU : 4 GiB Ram.

Running Unicorn you'll probably only want 1.5 instances per vCPU. Even a memory heavy Rails app is probably only going to utilise ~20% of the available memory.

Running threaded Puma, you probably want only a single process per vCPU and maybe 5-6 threads. In my apps running 5 threads per process typically increases memory of the process by 20%. So in that instance you'd only utilise 15% of the available memory on a M5 instance.

If you are having memory issues on Rails, then quick wins are upgrading your Ruby version. I saw 5-10% drop in memory usage with each of the major version 2.3.x -> 2.4.x -> 2.5.x.

Also if it is an old app, check you've not built up cruft in your Gemfile. Removing unused gems can be another quick win for reducing memory usage.