Choosing by evidence, not by seniority

The best referee is the person who saw you do the thing the new job needs, recently enough to still remember it. That is usually a direct supervisor, a project lead, or a tutor who marked your work — rarely the most senior name available to you.

Two are normally enough, and they should cover different ground: one who can speak to how you work day to day, and one who can speak to something harder — a deadline you rescued, a mistake you handled, a piece of work above your level. That pair is far stronger than two people agreeing you were pleasant.