Crystal is not ready for large scale deployment, which includes the fact that is not well supported as more widespread languages (e.g. on AWS).
Given that both fly.io and Heroku have working buildpacks for Crystal/Lucky, I’d say you can serve pretty heavy load before you’d need to reach for more specialized machines. Of course, that depends on the nature of your application as well. Maybe my understanding of 'large scale' is different from yours, though.
At large scale, I expect a programming language to support multithreading, in order to avoid the resource occupation of multiple processes. When one runs dozens of serving instances on a server, threads and processes make a significant difference in terms of memory requirements.
The experimental (therefore, not production-grade) support for parallelism has very poor performance, see their benchmark of the web server: https://crystal-lang.org/2019/09/06/parallelism-in-crystal.h....