The Pi 3B+ is adequate in its current form to run up to 16 extensions and 8 simultaneous calls. The Pi 4 is obviously a much more capable platform so it should be expected to be able to run more, although you will have to wait for our next announcements before we can give you some figures.
I would not recommend a full Raspbian installation (known as Pi OS now) since this includes a lot of other services and dependencies that will eat up more resources and disk space, as well as interact with the PBX in a way that may cause problems (keep it simple, bare essentials only). Single board computers are geared towards cost effectiveness and power efficiency, so a full OS would compromise that anyway if the intended use-case is the 3CX system.
The Raspberry Pi foundation has some good material if you want to learn how to use a Pi better (including SSH which is more than adequate when talking about a purpose-built PBX system).
https://www.raspberrypi.org/documentation/usage/terminal/README.md
Since you are interested in using a Pi, it is a very good learning tool to help you learn how to use Linux, and there are countless resources online that answer frequent questions when you use keywords such as Pi or Debian in your searches
Note: Pi OS or Raspbian, is version of Debian Linux for the Raspberry Pi computers, and almost everything you can search for Debian online is knowledge you can also use on a Pi.
Enjoy!