After exhausting the resources at nyc.gov and the Department of Education, I contacted several offices of the Department of Education for an answer. I was finally directed to the Press Office, where I was interrogated about why I wanted the information and who I was giving it to! In the end I was being bounced back and forth from Human Resources (which "doesn't give out information over the phone") to the Press Office, and so on. One person I spoke with told me she didn't know why anyone would have that information, yet clearly it exists somewhere, even if it is uncompiled.
The closest I came to an answer was the Vital Statistics for the 2001-2002 School Year, which lists the number of "other pedagogic personnel," including guidance counsellors.
It sounds like the figure of 300 is someone's guess, and, as these things go, it might not be incorrect. But to really find out, you are going to have to pressure the Press Office of the Department of Education for an answer. The number is (212) 374-5141