The minimum for an automatic bid is six teams (believe that is a NCAA-wide rule) and like ecemleafs said, that is one of the reasons the Big Ten is beginning hockey this fall. The CHA finished with four teams (keeping its auto bid through an exemption) and the old Great West also had four before dissolving.
There is no minimum in terms of conference size. Just having a conference without an autobid makes no sense.
Blog: First Round Bust: A Cast of Thousands celebrating a rather dodgy track record of Minnesota Wild Drafting.
"Will beats skill when skill doesn't have enough will."