How many scenes should be in each act of a play, and how many acts should a play have?
There are probably many ways of approaching this question; the following is simply my personal view.
Let's consider this as a problem in decomposition. I will start with the premise that the purpose of the play is to tell a story. (This is not always a valid premise but it should work for most cases).
Typically a story can be divided into 5 components or "arcs":
- Presentation of Initial State (sometimes called the Exposition or Stasis) in which the significant characters, the setting, and the genre are established.
- Rising Action in which something happens that unbalances the Initial State, a Response is invoked, one or more Twists ensue causing an increase in the Intensity or Significance of the change, and a Critical Choice emerges.
- Climax in which a previously unrecognized strength or weakness of the protagonist is revealed.
- Falling Action in which conflicts are resolved.
- Resolution which establishes the new norm state.
Such a division leads to (at least the possibility of) a 5 Act play structure. These are the typical plays of Shakespeare or Horace.
Depending upon the complexity of the story, the 5 arcs may be presented in a compressed form. Aristotle's 3 Act form would be typical of this:
...or, for a very simple story line with minimal textural development, a 1 Act play.
At the other extreme, you might have the Serial Story where each act or episode ends with an increasingly serious threat, leading to an effectively infinite series of acts.
As an aside, I personally do not hold with the view that acts should be defined based on how long the play-write can hold a audience captive before allowing them to get up and stretch.
Where do "scenes" fit into all of this?
I would define a scene in terms of 3 components: Formation, Interaction, Dissolution. Typically a group of actors come together (before the audience); actions occur or ideas are exchanged; the actors move off stage or out of focus.
How many scenes should there be in an act? Exactly the minimum number needed to fulfill the story function of the act.