How many celebrities is there in the world?

Although pretty much impossible to tell, we can always turn to Wikipedia.

In 2008 15% of articles were bases on "Biographies and persons" assuming each article was based on a notable person. In 2008 there was 2.25million articles wrote in English. Therefore, there was 337,500 famous people documented on Wikipedia. If we assume that that percentage has held true into 2012 then there should be 600,000 articles on famous people (written in English).

Sources:

http://en.wikipedia.org/wiki/File:Wikipedia_content_by_subject.png

http://en.wikipedia.org/wiki/File:EnwikipediaArt.PNG