The official UNESCO list is 962, though there are many sites that are of great importance that are not listed, such as Babylon. Sites are nominated, usually by the government of the country itself, and this can explain why some sites are not listed as there is not always the political will to go through the process of listing it and the subsequent upkeep and preservation demanded by a listing. As the burden falls upon the country of the site some critics have said the listing process is unfairly weighted against poorer countries who often can't afford to maintain the sites to the same standard as richer nations. Some countries wont put forward sites for this reason.
http://whc.unesco.org/en/list/ - a list and map of all current listed sites.