Current implementation is such that you sometimes get one bar, sometimes two, and it can’t be changed. Can this be added to the preferences as a drop down menu, or even to the DAW as a right-click menu on the metronome?
When you use “Record with Preroll” the duration is configurable in Preferences > Transport.
However that does not work directly at zero (or session-start).
Count-in clicks up to two bars, depending on where you start. It’s until “the bar after the current one”, and then starts rolling immediately. On the upside this works at zero, on the downside if you play a bit too early the first note may be list.