There is no "rule" as such. It has been only recently that the number
went from 8 to 12, but then the number of Div I teams has only been
growing gradually. Once upon a time, not too long ago, there were only
two conferences at DivI level, ECAC and WCHA. Progress seems pretty
good when taking a little longer term perspective. I don't know the
statistics, but it might be interesting to look at the number of teams
at all Div levels, especially I and III, over an appropriate number
of years.