It's pretty well-known for traditional television broadcasts, right? Shows are edited and even scripted specifically to provide the right amount of slots for ads.
It's much harder to measure television ad impressions than digital ad impressions.
Publishers charge for digital ad impressions by the 1000. It's easy to measure because usually they receive an HTTP GET request indicating the ad has been served.
For TV that uses traditional broadcasts you have to sample and scale. This is what Nielsen and other ACR companies do.
