R, 27 23 bytes

2-!length(readLines())

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Inspired by rturnbull's answer to Third time the charm, but the simplicity of the check makes it shorter.

Try it doubled, tripled, and quadrupled.

R, 27 25 bytes

1+!1-length(readLines())

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Inspired by rturnbull's answer to Third time the charm, but the simplicity of the check makes it shorter.

Doubled | Tripled | Quadrupled

Why this works:

readLines() will actually reads the source file itself rather than stdin. Hence, adding lines just increments the length() of the vector returned by readLines(). Therefore, we compute !(1-length()) to obtain 1 whenever length()==1 and 0 when length()!=1, adding one to have the desired effect.

R, 27 bytes

1+(length(readLines())==1)

Try it online!

Inspired by rturnbull's answer to a related question, but the simplicity of the check makes it a byte shorter.

Try it doubled, tripled, and quadrupled

R, 27 23 bytes

2-!length(readLines())

Try it online!

Inspired by rturnbull's answer to Third time the charm, but the simplicity of the check makes it shorter.

Try it doubled, tripled, and quadrupled.