Can I record a FaceTime audio call using my iPhone X on my Mac?
There is no option available natively in macOS to record a FaceTime audio call.
However this can be accomplished easily with the help of a 3rd party software.
As outlined in the blog post, you can record both a FaceTime audio as well as a regular phone call. However, make sure about the legality of the act. You may be required to inform the other party about the recording.
No affiliation whatsoever with the developer, they are very well known for the quality software they make.
In the upcoming macOS 10.14 Mojave (in public beta right now), the voice recording app comes to mac.
In theory you could use this to record a FaceTime audio call on your iPhone X. However as Nimesh mentioned, you might want to ask the other party first.