There have been 11
The NFL would have made a clean sweep if not for the BCS Championship game and game seven of the NBA Finals. Both of those games barely out-rated the AFC Wild Card matchup between the Bengals and Texans.
After the NFL playoffs you can see a pattern in the chart below. Championship games will typically draw an audience of 23-26 million viewers. After those games, there is a drop-off to the next group of big games which typically draw 14-17 million viewers...
Data via SportsMediaWatch.com