It is just a rough guess based on my experience as a grad student. The average professor at my university had 1-2 grad students at a time, and with 6 years to finish a PhD and about 35 years where a professor takes on grad students it works out to roughly 10. I wouldn't be surprised if the number was actually as high as 20 though, 10 is probably a low estimate. Another way to look at is that roughly half of people go on to do a 1st postdoc, half of those go on to do a 2nd and 3rd or 4th postdoc, half of those get a tenure track position and around 3/4 of people with a tenure track position actually get tenure. The problem with a career in physics these days is that a postdoc position is a terrible deal. As a graduate student you at least have job security and can stay in the same place for a good amount of time, not so as a postdoc.

It really depends on the field -- fields with labwork tend to have more PhDs per professor because there's an incentive to get cheap labor.