I heard on a radio show that it is statistically possible for there to be 6 undefeated teams after all the bowls and MNC are played. If that's true, what a huge mess for the BCS.
It's not really statistics, but you can still have 7 undefeated teams after the bowls.
In the ACC, SEC, and Big 10, you can only have one undefeated team. The Big 12 can have multiple undefeated teams, except the current undefeated teams all play each other, so only one from there. Stanford can be undefeated coming out of the PAC 12. Then Boise St. and Houston can both go undefeated.
Theoretically, you could then have the Big 10 or Stanford defeat a 1 loss team in the MNC. Then the ACC wins the Orange vs a team with a loss, then the SEC wins the Sugar vs. a team with a loss, Stanford or the Big 10 wins the Rose Bowl vs. a team with a loss, then the Big 12 wins the Fiesta Bowl vs. a team with a loss, and Boise St. and Houston both miss the BCS and win other bowls.
... However, this is incredibly unlikely because a 1 loss team would not make the MNC with 7 undefeated teams out there. Also, Boise St. would not be snubbed by the BCS if they went undefeated. And in a world with 7 undefeated teams, the SEC and Big 12 would make the MNC, leaving Stanford vs. the Big 10 in the Rose Bowl. And all of that assumes 7 teams can get through the regular season and conference championship games without a loss.