How many measures does the most common form of American popular song contain?

The most common form of American popular song is often structured in what's called the "32-bar form," which typically consists of 32 measures (bars) of music. This form is usually divided into four sections: the AABA structure, with each section typically being 8 measures long, adding up to 32 measures in total. So, when referring to the most common form, American popular songs usually contain 32 measures.

Extra: The AABA 32-bar form has been a staple of American music, particularly in the mid-20th century. This structure involves three sections: two identical sections (A and A), a contrasting section (B), and a return to the first section (A). Each 'A' section often ends with a similar melody and lyrics, which helps to establish the song's theme and makes it catchy. The 'B' section—also known as the bridge—provides a contrast either melodically, harmonically, or lyrically.

American popular songs, especially those from the Tin Pan Alley era and many Broadway show tunes, have used this format extensively. The 32-bar form is also a common structure for jazz standards and is widely used in jazz improvisation, where soloists will often take turns improvising over the 32-bar form of a song. It's a very clear and predictable form that allows both the performer and the listener to anticipate the flow of the song, creating a satisfying musical experience. Owing to its popularity, many iconic songs like "Over the Rainbow" and "I Got Rhythm" are structured in the 32-bar form.