How many books existed before the printing press?

Before Gutenberg (about 1450), books were copied by hand, and total output across medieval Western Europe is estimated at roughly 10 million manuscripts over a thousand years. In the first 50 years of printing, about 20 million copies were produced; in the 16th century, 150 to 200 million. Google estimated about 130 million distinct book titles in 2010.

Source: Buringh & van Zanden (2009), Journal of Economic History; Febvre & Martin, The Coming of the Book (1976)