On June 24, U.S. Senator Sam Brownback was asked whether he would agree to inclusive general election debates, if he becomes the Republican nominee. He expressed fear that there would be too many candidates, and said he would need to think about it. This occurred in Roland, Iowa. Thanks to Larry Reinsch for having raised this issue with Senator Brownback. Reinsch is also the person who asked Senator Hillary Clinton and Senator Barack Obama the same question earlier this month.
It should be noted that in all U.S. history, there has never been a presidential general election with more than 7 presidential candidates who were theoretically able to be elected. Specifically, there has never been an election with more than 7 presidential candidates who were on the ballot in states containing a majority of the electoral college.
There were no government-printed ballots before 1889, so for the elections before then, this statement refers to the ability of various presidential candidates to circulate private ballots. There were never more than 7 such candidates in that period either. In the period 1856 to the present, the average number of presidential candidates who could theoretically have been elected has only been 4.3 candidates.