i think there is no counting for posting classifieds you post all classifieds but dont post same classifieds no of times
dear no limit but every day doing different activity like one day doing directory submission and second day doing social book marking.
1 ad is spam if you are trying to sell something in a category that doesn't fit. Make sure the site allows the type of ads you are trying to add to the site and be sure you put it in the right category. Follow the sites rules and there is no limit to the number of ads you can place.
Do classifieds submission only for high pr sites not for 0 pr sites, you can also share your classified ad links with lot of other social bookmarking sites which can boost your search engine rankings.