Ad servers keep track of the number of times the graphic (or other asset) for a particular campaign was served. Lots of other "pixels" also get bundled into the calls depending on what kinds of ad tech the advertiser, publisher, or middlemen are running.

How many people (living, breathing, checkbook-possessing people) actually see an ad is an entirely different question from how many it is served to, and there is (of course) a whole 'nother area of ad tech companies working in that space.