I'm one of those hoarders who grabs big archives of stuff and stores them away, so I have many. How many, I have no idea. I'd say at least 30 GB, much of which is multi-megabyte PDF versions. The text files will be well under 1 MB on average. So, overall-- probably well over 10,000, probably less than 50,000.
Edit-- for anyone who doesn't know, here's a quick (assuming that you have broadband-- if you have dial-up it would be a 3 week download) way to have around 30,000 books (I don't have this download, it doesn't count in the number I gave.)
http://www.gutenberg.org/wiki/Gutenb...nd_DVD_Project