Most server backends can easily scale horizontally, including Ruby on Rails, Node.js, and Elixir. The only requirement is that you don't keep any "state" in your server code. A novice programmer can make this mistake in any language, including Elixir.

Read about the "Twelve-Factor App": https://12factor.net/

The main bottleneck is usually your database, which can be tricky to scale. You set up things like read slaves and sharding.