I am currently a candidate for a few jobs, and have a few campus interviews.
How many candidates are typically invited for a campus interview?
I am mostly interested in answers for positions for the range of positions, tenure-track, non-tenure track, lecturer positions, and postdocs.
For tenure-track positions my department generally invites 3 candidates for on-site interviews after doing phone screenings. This is what I have seen from a lot of universities in the US.
Other types of positions don't have standard rules. Non-tenure track and adjuct positions are filled as needed, don't involve as rigorous of interviews, and may never be brought on-site. Postdocs are usually hired by the individual PI rather than the department.