You might be losing no ads, several or hundreds as it is in my case. The only way to be able to access those statistics is to setup alternate ads and rely on the stats of the company that is providing the stats. The reason why you should setup alternate ads in Google Adsense is therefor twofold. First you probably would like to know how many PSAs are displayed and you would then like to monetize them instead of handing them out to Google for their public service announcements.

Setting up alternate ads is relatively easy. You first need to have some alternate ad code from a company such as Amazon, Chitika or Auction Ads and save the code into a normal text document. Rename the document to alternate.htm and upload it to the root of your website.

Now add the line google_alternate_ad_url = "http://www.yoururl.extension/alternate.htm"; to your Google Adsense code and the alternate ads should appear on every page where Google is unable to provide advertisement for.

After setting them up you can start analyzing the number of impressions that you receive from that backup network and tweak it until you are satisfied with the results.