Apple announced today that iTunes has become number two music retailer in the US, behind only Wal-Mart, based on the latest data from the NPD Group.

Apple also announced that there are now over 50 million iTunes Store customers. iTunes has sold over four billion songs, with an incredible 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.

“We’d like to thank the over 50 million music lovers who have helped the iTunes Store reach this incredible milestone,” said Eddy Cue, Apple’s vice president of iTunes. “We continue to add great new features like iTunes Movie Rentals to give our customers even more reason to love iTunes.”