I would like to know how many users take part in Yahoo! Answers and
especially in the German version, which is called Yahoo! Clever

It sounds like a good question for Yahoo! Answers. ;-)

As to the first comment, it kind of depends on your view of Yahoo!
Answers. As one person put it:

"Take a look at the level of questions and answers (see "browse
resolved questions"). Stupid, and weird."

Anyway, if it's any help, there have been 10 million answers posted in
the first five months of the existence of Yahoo! Answers:

http://www.ysearchblog.com/archives/000303.html

and I believe that Yahoo! has around 200 million account holders.

Participant numbers will be quite large.

Have a good day.

Thank you for your comments! I already asked in Yahoo! Answers, but
the users don't have any idea. Also, I found the 10 million answers.
But the number of members is ungoogable for me.

Yahoo Answers appears (at least based on the questions and comments)
to be frequented mostly by giggly preteens and the mildly retarded.
You would think that such a transparent attempt to copy the kids at
Google would at least have some improvement. It's more like a chatroom
than a question/answer forum. Not that GA doesn't have the occasional
goofy question, but YA made me feel like I was in the middle of a
gaggle of screaming 11-year-old girls.

I suspect that my comment here under the Yahoo Answers' system would
count as an answer - that?s how they probably get 10 million answers.
There is often 10 to 40 answers per question.

No comments on Yahoo, just Answers.
However, one Answer is deemed a winner, gets a reward?, possibly a lolly bag.