A single adsense code can be used on multiple pages..?Or we have to select new ad for every page/place on a same website.
Of course, you can use it as many times as you like. As long as the ad design matches that particular page you're fine. The only issue you would have is being able to track performance of certain pages, or certain spots, in which case you need a new ad for each spot, with a unique name, then you can see in stats and compare different ads across your site.
the owner of this forum doesn't make a new ad code every time a new thread is created. who would want that job anyway?