Complex sentence: "Sad Eyes" was one of the few non-disco number-one hits on the Hot 100 during 1979--during the first 9 months of the year, 13 of the year's 16 chart-toppers were disco or had strong disco influences.
Split into simpler sentences: Sad Eyes was one of the few non-disco number-one hits on the Hot 100 during 1979. During the first 10 months of the year, 13 of the year's 16 chart-toppers to the time when "Sad Eyes" reached the top were disco or had strong disco influences.