Apple Computer today announced that iTunes is now the number two music retailer in the United States, second only to Wal-Mart, based on the latest data from the NPD Group. Apple also disclosed that there are now over 50 million iTunes Store customers, who have bought over four billion songs (20 million songs sold on Christmas Day 2007 alone) from a catalogue of over six million songs from all of the major and thousands of independent labels.

Today’s number-two retailer ranking is based on data from market research firm the NPD Group‚Äôs MusicWatch survey, which captures consumer-reported past week unit purchases. The research census counts one CD as representing 12 tracks, excluding wireless transactions. The iTunes Music Store became the second-largest music retailer in the US after Wal-Mart, based on the amount of music sold during 2007.