Ralph Nader has again called for more inclusive general election presidential debates. See this story. The case for more inclusive general election debates is stronger than ever, because the 8-candidate and 9-candidate Republican presidential debates this season have attracted so many viewers and so much commentary. These Republican debates are strong evidence that multi-candidate debates are useful
In the entire history of the United States, there has never been a general presidential election with more than 7 candidates who could theoretically have won the election. For a list of the presidential candidates in every general election 1856-2004 who were on the ballot in enough states to theoretically win the election, see here. “On the ballot” in the years before 1892 (in other words, in the years before there were government-printed ballots) means candidates who went to the trouble to nominate slates of presidential elector candidates, print up their names on privately-printed ballots, and distribute them to voters who wanted such a ballot, in states containing a majority of the electoral vote.
The link to the chart for 1856-2004 does not include 2008, but in 2008 there were six presidential candidates who were on the ballot in states with a majority of the electoral vote: Obama, McCain, Barr, Nader, Baldwin, and McKinney.