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.