According to this story, the December 15, 2015 Republican presidential debate had the third highest viewership of any televised presidential primary debate in U.S. history. This is more evidence that having as many as nine candidates does not limit the usefulness or success of a debate.
In the entire history of the U.S., there have never been a general election with more than seven presidential candidates who had enough presidential elector candidates to theoretically win the election. The position of the Commission on Presidential Debates, that its debates should be limited to two candidates, is no longer tenable. Thanks to PoliticalWire for the link.