I need a thank you letter so that I could send the same to all visitors after exhibition

An art show at a gallery?
A business expo?
A Tupperware or Avon party?

I'm guessing a business expo, but I can see where you are having a tough time writing a simple letter if you just assume that we know what.
Close to 90% of your letters will be tossed in the trash unread, the rest skimmed for about 4 seconds. Therefore keep it SHORT. Then provide a rate chart and a discount. If they already took your rate chart, then a sampling of the most popular items or services. Money is the bottom line. Keep it all to one page, or better yet, a large postcard (within postal limit on size) so they don't even have to open the envelope. A more expensive way is a folded card.

This is for Thank you note for Post exhibition. To send it to all the visitors who visited our booth during the event.