How do I invoke a command by tapping 2 (or 3 or 4) fingers on the trackpad?
As far as I know, you can't. KM offers gesture triggers, which are pretty cool. But if you want tapping on the trackpad as a trigger you need an app like BTT.
Look for the posts by @appleianer - they’re the maestro when it comes to Better Touch Tool, trackpad and Keyboard Maestro