Given the current requirement that divisions must play a round robin
schedule, I am not sure 18 members would work in football. I believe the reason that most "experts" place the cap at 16 members is because not every conference is willing to play more than 8 conference games in football. Regardless whether a conference decides to play 8, 9, or even 10 conferences games in football, the conference schedule will include at least 1 cross-division game. If an 18 member conference decides to only play 8 conference games, then that conference will not have any cross-division games on its schedule.
A common defense used by the "experts" to support the cap being at 16 is the idea that once the divisions stop playing each other during the regular season as conference games, then the CCG would be the only thing to tie the two divisions together. As a result it would make it easier for people to view the divisions as being (more or less) their own conference. Once that happens then it could lead to instability within the conference and make it easier for 1 division to decide to break away to form their own conference.
Even though the WAC had other issues going for it when it decided to expand to 16 members and within a couple years 8 schools decided to leave to form the MWC, I do believe the power conferences use it as a reference point because they don't want to see history repeat itself with their conference. I believe DII and DIII may have a conference with 16 members. However, I don't think either division level has a conference with 18 or 20 members.