Okay, I'm at work and I'm having my coffee, so I'll take a little time to explain infringement analysis.
Copyright protection is authorized in Article I, Section 8 of the Constitution, and reserves
exclusive rights to authors (and inventors) in their works of authorship (and inventions) for limited periods of time.
Copyright is infringed when one of the reserved rights of copyright is violated. These are found in 17 USC § 106. I'll address only the right to make copies.
Copyright is infringed when someone copies a protected work. Period. As I've noted, there is no magic quantum that can be copied before liability attaches -- if you've copied, you've infringed.
Copyright infringement requires proof of two things: ownership of a valid copyright and unlawful copying by the defendant.
Unlawful copying can be proven in one of two ways. Direct evidence of copying, e.g. someone testifies, "I saw him copying," is the easiest, but that kind of evidence is rarely available. It is therefore usually necessary to prove unlawful copying through indirect or circumstantial evidence. The courts have evolved an analytical framework for determining unlawful copying from circumstantial evidence.
Unlawful copying is evaluated by two factors: (1) access to
the original work, and (2) substantial similarity of the infringed work to
the original work. These are measured on a sliding scale, i.e. if there is a lot of access, relatively less evidence of substantial similarity is required to prove unlawful copying, and vice versa. Note, however, that if there is
no access to the original, e.g. I wrote a lyric, put it in my drawer and never showed it to anyone, there can be no unlawful copying. In that instance, each author has an enforceable and independent copyright in their respective works.
Substantial similarity is evaluated by a two-part test: objective substantial similarity and subjective substantial similarity. Objective substantial similarity involves
expert testimony and literal comparison between the original and the accused works. Subjective substantial similarity simply asks, "would a reasonable person, upon seeing the accused work, think it was copied from the original?"
If both objective and subjective substantial similarity are proven by a preponderance of the evidence AND access to
the original is proven by a preponderance of the evidence, upon proof of a valid copyright, and absent any affirmative defenses to copyright infringement, e.g. fair use, the defendant is liable for infringement.
And that's the way it works. It does not involve counting a magic number of notes or measures.