Complex sentence: He was very popular in the 1980s and early 1990s, where he has sold more than a million albums, who also has written or co-written 21 number one singles, since his debut, as a solo artist, in 1978.
Split into simpler sentences: He was very popular in the 1980s and early 1990s, during which he sold more than a million albums. He also has written or co-written 21 number one singles, since his debut, as a solo artist, in 1978.