PhD students like to talk about the fact that there are far more new doctors of philosophy than positions as tenure-track faculty or researchers, so anyone who wants a job like that has to follow a series of very specific and demanding steps, with a high chance of finding themself stuck in a poorly paid, overworked position as a sessional instructor or post-doctoral researcher. Unfortunately, hard numbers are hard to come by, and naturally the people who are very successful or very unhappy have the loudest voices. The people who are most active in complaining about the problem tend to be Americans, and the situation in that country has some special features. Back in 2013, I estimated that about four people got a PhD in history in Canada for every tenured professor who retired, and made some choices accordingly.
Recently, the University of British Columbia published a survey of 3,805 students who graduated UBC with a PhD between 2003 and 2015. Through a combination of mail, email, and online searches, they were able to find some information about 91% of these students. A summary is posted at http://outcomes.grad.ubc.ca/