How many PhD students can one supervisor supervise?

Ross Woods, with thanks to Yαsir Rαshεεd 2023

I've supervised about a dozen thesis students at once without difficulty, but they weren't PhD students. One famous academic reportedly had about 30 PhD students at any one time. The number of students depends on many factors. Here's my list:

  1. How well prepared are students in writing and methodology?
    1. If they are very well prepared, the supervisor has much less to do.
    2. If they are poorly prepared, the supervisor has lots of work to do, probably for mediocre results.
    3. Some programs do coursework during which students choose their topics, then write a literature review and methodology plan before they actually qualify to start the dissertation. The supervisor has much less to do.
  2. How good are the students? Self-starters almost supervise themselves. Weak students need more help.
  3. How much supervision experience does the supervisor have?
  4. How many supervisors for each student? The work is shared when institutions have committees, so a supervisor can have more students. The commitee chair has more to do than other committee members, but the others might contribute expertise in (for example) methodology, statistics, and dissertation presentation.
  5. At what stage are students at the time? Students need lots of help at the early and final stages. During the middle stages, they are busy gathering data and need less supervision.
  6. What kind of pressure are supervisors under to have a high pass rate? if they are under excessive pressure, they do extra work to support weaker students and get them through. This kind of pressure can come from funders and or accreditors. It can also come when a foreign government sponsors a weak student to study overseas.
  7. Do students use supervisors as a proof-reading and editing service? Supervisors do lots of extra work if they get trapped into this. (It's probably justified for first cohorts in a new program but not for existing programs.)
  8. What's the supervisor's style? It can vary from a light touch where they give as little supervision as possible, even to the point of neglect. Some have a style that is little more than daily hovering.
  9. What is the supervisors' workload in other responsibiities, such as teaching and administration?
  10. If the supervisor's expertise and supervision is restricted to a specific area, they can supervise more students in that particular area.
  11. Students might need more support if they are working in a second language or come from another culture, even if they are very good. This might reduce the number of supervisees.