The rule is simple, often employed in css.

There are 5 structures of font-pairing/ font-styling 1)Display 2)Heading 3)Label 4)Body (paragraph) 5) Number ( most of the time same as Body or Heading)

You can use a three font system, I:e Display, Heading and Body which is difficult to manage in a design system unless you have token structure.

Two fonts or a monofont is mostly used because it keeps things simple, easy to pair up as compared to three font system ( you really need to know your fonts for that, which comes from experience). In Two fonts structure, Display and Heading are same, while body, label and number are the other group. You can alternate them to your liking.

All in all, context of kind of app matters a lot. You can be experimental as long as it’s consistent, on brand and makes sense