How many interviews is best?

This is going to vary from job to job, and candidate to candidate. But a good rule of thumb, when you get to the decision round, is to divide the candidate's past experience into manageable chunks, where each interviewer covers one or two jobs from the candidate's past in their interview.

For candidates with a long career history, you may want to focus your interviews on the most recent, relevant experiences, with a light touch on older, less relevant roles.

Similarly, for entry-level roles and candidates with minimal job experience, you may want to focus some of the interview time on their experiences in high school, university or graduate school. See Should I ask a candidate about their time in high school?