[marceloabsousa]

- There are a fixed number of funded PhD spots but there's really no limit to the number of PhD spots in a faculty. I don't know if this was the case but if you can fund your own PhD, very few universities will refuse your admission. Anyway, PhDs are extremely cheap labor - most of the research money is spent on materials/hardware and travel expenses for PIs.

- The vast majority of PhDs don't do anything productive with their research topic or work in their research field later on. Doing a PhD is about advancing human knowledge and a lot of personal growth - it has very little to do with productivity. The current trend is that you get burned out and sick of your research area and decide to do something totally different. There are very few areas where you do a PhD for credentials or to advance in your career.

[Mediterraneo10]

The amount of possible PhD positions in a department is limited by the number of faculty, because each student needs a supervisor. A supervisor can only supervise so many students at once before he or she feels overworked, and each PhD student to supervise may distract from that scholar's own research.

[marceloabsousa]

Sure it's a finite number but it's not uncommon for a faculty member to have 5+ PhD students. It all depends on the number of post-docs or senior PhDs they have in the group. The major limiting factor is really money - if a faculty member gets a $5M grant, they will hire as many PhDs as they want or even get some faculty hire for the project.