Well... "There are 30 teams in the NBA, but the league has worked out that these can be transported with 13 aircraft." Simple math dictates that only 15 teams are traveling at any given time, and with several in the same city and/or close enough to possibly bus it between East Coast locales, the logic seems sound that 13 planes can be shared amongst all the teams. Portland will presumably be using the highest percentage of the "motor pool."
