There a total of 11 Ads per page on Google Search. They are sorted by 'rank' based on the quality of the ad, the amount bid and the landing page quality.
Google usually displays 8 Ads in the search results, 3 ads in the top and 8 ads in the bottom.
Google lists ads depends on the user search queries and advertisers competing for those keywords. If there is no advertiser then obviously you wont see any ads.