For n=1, every point in the critical divisor has degree $≤d-1$, where $d$ is the degree of the map, and the total degree of the critical divisor is $2d−2$, and any such divisor can occur.

To state it without degree, the number $m$ of points (counting multiplicity) is even and multiplicity of each point is at most $m$.

This result is well-known and is easy to prove, but here is a recent reference:

https://www.math.ucdavis.edu/∼kapovich/EPR/covers.pdf

For n=1, every point in the critical divisor has degree $≤d-1$, where $d$ is the degree of the map, and the total degree of the critical divisor is $2d−2$, and any such divisor can occur.

To state it without degree, the number $m$ of points (counting multiplicity) is even and multiplicity of each point is at most $m$, and this is a complete description of critical divisors.

This result is well-known and is easy to prove, but here is a recent reference:

https://www.math.ucdavis.edu/∼kapovich/EPR/covers.pdf