According to avianweb.com:

"Budgerigars can be taught to speak and whistle tunes. In fact, they are believed to be the best talkers of all birds. They can learn to pronounce hundreds of words and phrases. In fact, one California budgie is said to have had a vocabulary of 1,728 words by the time he died in January 1994. Another budgie called "Sparky Williams" had a repertoire of eight nursery rhymes, 360 phrases, and a vocabulary of over 550 words. In fact, this little budgie became a star and 20,000 copies of his records were sold by the time he died in 1962."