Number of librarians census

The U.S. Census first collected data on librarians in 1880, four years after ALA’s founding. They only counted 636 librarians nationwide. Indeed, one respondent reported on his census form that he was the “Librarian of Congress.” The U.S. Census, which became organized as a permanent Bureau in 1902, can be used to track the growth of the library profession.
Librarians in the United States, 1880–2009. (2011). Cambridge: Oxford University Press.
http://blog.oup.com/2011/06/librarian-census/