The out-of-the-box theme has 2 ad areas which you can set up from within the ClassiPress admin dashboard, this is the Header Ad (468x60), and Single Ad (336x280) - which is the ad contained in the ad page itself. You can also configure a widget for additional 125x125 ads in the sidebar. If you wanted to, you could customize the theme for additional ad spaces, and if you wanted different ads on each page, that again would require customization on your part.
