The announcement says, "over 20 tracks new to the Rock Band franchise"; it's not like they said, "more than 20, less than N" . Of course, your scenario seems unlikely for XBL Arcade prices.
Mike Scott, San Diego, CA, USA (XBL: MikeHellion, PSN: MarcHellion)
 Katy Perry
 Dream Theater
They always play Threshold on the radio, but Harmonix would probably cut it out for the same reasons Space Intro wasn't part of Fly Like an Eagle.