It looks like the number of ad positions they will maintain per page. I seem to only find two per page at certain times. I guess the Freshtunes box they consider to be one of the 3 ad positions.
They also have a pay per click section running down the right side sometimes. The average price per click is 43 cents according to AdBrite. Not sure how you begin to calculate this number based on preset advertisement limits most companies use for this type of advertisement.