I estimated the amount of graduate math students and especially the amount of math students in general from a theoretical point of view in an analogeous way like the [Fermi-Problem][1].

 - There are about $ 8*10^9 $ people living right now.  
 - About $ \frac{1}{5} $ of the people are aged between 18 and 28, let's call them young adults.
 - About $ \frac{1}{10} $ of young adults is studying.
 - About $ \frac{1}{50} $ of students study math.
 - About $ \frac{1}{5} $ of math students is studying at graduate level.

This gives $ 3.2*10^6 $ people who study math and thus $ 640,000 $ graduate math students.

  [1]: https://en.wikipedia.org/wiki/Fermi_problem