A search on Yahoo indicates about 15,000 categories of -- or tags for -- Yedda questions so far.

Here is my Yahoo search for Yedda categories.

15,000 categories strikes me as a reasonable number, since it's about twice the number of Yedda questions.  (There are currently 1066 pages of questions, at eight questions per page -- or approximately 8500 questions.)  I'd be surprised if there were an average of, say, three original tags per question.  Two seems a more plausible number.