I have a powerful computer and a fast internet connection. I'd like to support the Tor network by operating a relay. I've seen that there are relay packages and instructions for various versions of Linux, BSD and other Unixes, and Windows - but not MacOS. Why not? Or have I missed something?
There are currently 6 relays running on macOS in the consensus, but I don't think it's an officially supported configuration. There are not many people who run Mac servers, so it probably isn't worth the extra effort to build and maintain a Mac-specific package. I don't think there would be any issues running tor though, just install tor (through homebrew, compile from source, etc), configure your torrc file like on other platforms, and set your Mac to run tor at startup. If you run into any issues, ask a new question here and someone might be able to help.