so the math is:

1.800.000 / 50.000 (avg) is 36 persons, somewhere in the ballpark range i mentioned

Well, no. That money is the total grant sum, to be spent over multiple years, and PhDs are usually funded in full - in Germany a PhD takes around 5 years. Moreover, the money a PhD student gets after tax, is not the same as the money spent overall from the grant, see my other comment below.

In Europe a PhD student is a multiyear commitment, with a bunch of externalities, they are much simpler to manage as discrete units, and thus are funded as such.