He indicated that the expansion refers to both retail titles and to the downloadable content available for the games. He intends to bring more developers and more hardware suppliers to the party in order to accomplish this goal.
While no details were apparently released my unofficial count shows that there are currently six Guitar Hero titles available for purchase right now, counting Guitar Hero: World Tour which ships in October 2008. That could make twelve titles available in 2009 and 18 for 2010 if we understand Griffith correctly.