How many candidates are usually invited for a campus interview?
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.