How many number-one hit singles in the US did the producer of the album Harbor produce?
Harbor (album): Harbor is the seventh original studio album by American folk rock trio America, released by Warner Bros. Harbor (album):  The album was produced by George Martin. George Martin: Sir George Henry Martin {'1': ", '2': ", '3': ", '4': "} (3 January 19268 March 2016) was an English record producer, arranger, composer, conductor, audio engineer, and musician. George Martin:  Martin produced 30 number-one hit singles in the United Kingdom and 23 number-one hits in the United States.
23