In this post Jim talks about how to use Dramatica to come up with first 21, then 28-32 structural beats of a story:
I think for a stage play you could use the exact same process, considering that many of the beats will occur within the same scene. This happens naturally as you weave the throughlines together anyway. Plus, a story driver or other beat which takes place offstage can be communicated very quickly in a given scene – “did you hear the rebels bombed the port?” – allowing the rest of the scene to address the next beat(s).
If you average two beats per scene then using Jim’s formula you end up with 14-16 scenes.