I need a listing for business conventions held in the United States
with number of attendees

Dear Sabine, 

Your question is about to expire without having been answered, and you
might be disappointed. 

One possible reason for that is the pricing you put on your question,
given the fact that you ask for collection of data that might be not
readily available, or that is collected only by professional
market-research firms.

In addition, here is a link to guidelines about pricing your question,
in the pricing guide: https://answers.google.com/answers/pricing.html
- I hope this would help you price your next questions on Google
Answers.