Question: How many number-one hit singles in the US did the producer of the album Harbor produce?

Context paragraphs:
Harbor (album): Harbor is the seventh original studio album by American folk rock trio America, released by Warner Bros.  Records in February 1977.  It was the last to feature Dan Peek, who embarked on a solo Christian career shortly after the album's release.  The album was produced by George Martin.

Eien no Tsubasa: "Eien no Tsubasa" is the forty-third single by B'z, released on May 9, 2007.  It reached number-one on the weekly Oricon Singles Chart, one of B'z many number-one singles in Oricon charts.

Usher discography: American singer Usher has released eight studio albums, nine compilation albums, eight extended plays, and fifty-three singles (including eleven as a featured artist).  His music has been released on the LaFace, Arista, Jive, and RCA record labels.  23 million of his albums were shipped in the United States, and sold over 43 million albums worldwide, leading to a combined 75 million records and making him one of the best selling music artists.  He also has 9 Hot 100 number-one singles (all as a lead) and 18 Hot 100 top-ten singles.  In 1994, Usher released his self-titled debut album in North America, producing three singles that had moderate chart success, and the album sold more than 500,000 copies.  His follow-up 1997 album "My Way" sold 8 million copies, becoming his breakthrough album.  It was certified six-times platinum in the US, and spawned three successful singles, including his first US "Billboard" Hot 100 number-one hit "Nice & Slow".  Usher's success continued in 2001 with his third studio album "8701".  It debuted at number four on the "Billboard" 200.  The album produced two number-one singles—"U Remind Me" and "U Got It Bad".  It has sold more than 4.7 million copies and has been certified four-times platinum in the US.  Its worldwide sales stand at over 8 million.

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.  He was referred to as the "Fifth Beatle", including by Paul McCartney, in reference to his extensive involvement on each of the Beatles' original albums.  Martin produced 30 number-one hit singles in the United Kingdom and 23 number-one hits in the United States.

Home (B'z song): "Home" is the twenty-fifth single by B'z, released on July 8, 1998.  This song is one of B'z many number-one singles in Oricon weekly chart.  Japanese rock band L'Arc-en-Ciel released three singles in the same day.  Although L'Arc-en-Ciel's three singles—"Honey", "Shinshoku (Lose Control)" and "Kasō"—passed the first week sales of 500,000 copies each and were ranked at number two, three and four respectively, the single "Home" managed to debut at the number-one position.  The single sold over 961,000 copies according to Oricon.  While they won "the artist of the year award", the song was elected as one of "songs of the year" at the 13th Japan Gold Disc Award.

John Farrar: John Clifford Farrar ( ; born 8 November 1945) is an Australian-born music producer, songwriter, arranger, singer and guitarist.  As a musician, Farrar is a former member of several rock and roll groups including The Mustangs (1963–64), The Strangers (1964–70), Marvin, Welch & Farrar (1970–73), and The Shadows (1973–76); in 1980 he released a solo eponymous album.  As a songwriter and producer he worked with Olivia Newton-John from 1971 to 1989.  He wrote her number-one hit singles: "Have You Never Been Mellow" (1975), "You're the One That I Want" (1978 duet with John Travolta), "Hopelessly Devoted to You" (1978), and "Magic" (1980).  He also produced the majority of her recorded material during that time including her number-one albums, "If You Love Me, Let Me Know" (1974), "Have You Never Been Mellow" (1975) and "Olivia's Greatest Hits Vol.  2" (1982); and he was a co-producer of "" (1978) – the soundtrack for the film of the same name.  Farrar also produced Newton-John's first United States number-one hit single, "I Honestly Love You", which was awarded the Grammy Award for Record of the Year in 1975.  In 1969 Farrar married fellow Australian singer, Pat Carroll – formerly Newton-John's singing partner.  In July 1970 Farrar and Carroll relocated to the United Kingdom and from late 1975 they have resided in the US.  They are the parents of Sam Farrar (Phantom Planet bassist and Maroon 5 touring member) and Max Farrar (Golden Ghost guitarist and keyboardist).

James D-Train Williams: James Nelson Williams (born January 21, 1962), known by the stage name D Train, is an American singer, songwriter, instrumentalist, producer and actor.  He rose to fame in the early 1980s with the release of his first album “You're the One for Me” (working with songwriter/producer Hubert Eaves III initially as a duo), which spawned his first US "Billboard" Dance chart number-one hit, "You're the One for Me".  His debut album (under the Prelude Records label) would also chart with the hits "Keep On" and the much covered Burt Bacharach hit "Walk on By".  He would go on to work with Eaves producing hits on the follow-up albums "Music" and "Something's On Your Mind" before being signed to Columbia Records, producing more hit singles on the albums "Miracles of the Heart" and "In Your Eyes".  The two parted ways in 1988 and D Train went on to perform on his own label, releasing albums such as "701 Franklin Ave.".

Please Don't Go (KC and the Sunshine Band song): "Please Don't Go" is a song recorded and released in 1979 on the KC and the Sunshine Band album "Do You Wanna Go Party".  Originally written in the key of D flat, the song was the band's first love ballad, in which the subject pleads for a second chance.  Shortly after the song's one-week run at number one, the group broke up and Harry Wayne Casey went solo.  The song was a number-one hit on the Australian ARIA Charts, the band's fifth and final number-one hit on "Billboard" Hot 100 charts, and the first number-one hit of the 1980s.  As the band was known as a pioneer of the disco genre, the song was eerily released on the same day as Disco Demolition Night in Chicago, Illinois.

Ichibu to Zenbu/Dive: "Ichibu to Zenbu/Dive" (イチブトゼンブ／DIVE , lit.  "Part and All/Dive" ) is the forty-sixth single by B'z, released on August 5, 2009 as a double A-side.  This is one of the many number-one singles by B'z at Oricon charts.  It was a long hit, staying in the top 5 and the top 10 for 8 consecutives weeks.  Chad Smith, from Red Hot Chili Peppers, and Juan Alderete, from The Mars Volta, guest appeared on this single on the drums and the bass guitar, respectively.  Two music videos were shot for the A-sides.

Walls and Bridges: Walls and Bridges is the fifth studio album by English musician John Lennon.  It was issued by Apple Records on 26 September 1974 in the United States and on 4 October in the United Kingdom.  Written, recorded and released during his 18-month separation from Yoko Ono, the album captured Lennon in the midst of his "Lost Weekend".  "Walls and Bridges" was an American "Billboard" number-one album and featured two hit singles, "Whatever Gets You thru the Night" and "#9 Dream".  The first of these was Lennon's first number-one hit in the United States as a solo artist, and his only chart-topping single in either the US or Britain during his lifetime.

Supporting paragraphs: Harbor (album) George Martin
Answer: 23