I haven't seen 20,000 documents reported anywhere, or perhaps I just missed it. Do you have a link to an article mentioning this? Thanks.
EDIT: Possibly http://www.nytimes.com/2013/06/11/us/how-edward-j-snowden-or... "Mr. Snowden has now turned over archives of “thousands” of documents, according to Mr. Greenwald, and “dozens” are newsworthy."