Apple today announced that iTunes is now the number two music retailer in the US, behind only Wal-Mart, based on the latest data from the NPD Group. The company also announced that there are now over 50 million iTunes Store customers. iTunes has sold over four billion songs, with 20 million songs sold on Christmas Day 2007 alone, and offers the world’s largest music catalog of over six million songs from all of the major and thousands of independent labels, says Apple.