Isn't 18 the right number for the Alliance?
I've always heard that 9 teams is the perfect number for a football conference, because each team can play 4 conference games at home and 4 away each season. It's the perfect balance.
With this theory in mind, it seems it would be favorable for both sides (MWC & CUSA) to have 9 members each, for a total of 18. Adding a couple of "crossover" games each season (1 home, 1 away) will take them to a total of 10 scheduled games, leaving 2 or 3 each season for OOC.
HD