I wondered about the number of papers published by mathematicians because I heard that they publish less than other academics or scientist. It would be nice if you could distinguish between the number of papers published by PhDs, Postdocs or Professors.
This varies on a lot of things (e.g., by field and what kind of institution someone is at), however, there have been various studies. For instance Figure VII (p70) of this study (which seems a bit strange to me) graphs the annual number of papers per mathematician and their most recent data (2005-2008) appears to put the average just under 0.7 for "high-exposure mathematicians" and 0.2 for "low-exposure mathematicians."
That said, things vary a lot by area and what one considers as "research active." I would guess that it rather pure, theoretical areas (which I am most familiar with), among people I consider professors research active 1-2 papers/year is pretty typical. For postdocs I imagine the average is < 1/year (including those who are not "research successful") and for US PhD students < 0.2/year (most PhD students probably don't have anything published by the time they graduate, though they may have 1-2 preprints).