Complex sentence: It was her biggest hit, reaching Number 1 on the "Billboard" Hot 100 singles chart in December 1971 and January 1972 and it reached Number 1 in Canada and Australia and Number 4 in the UK charts.
Split into simpler sentences: It was her biggest hit, reaching Number 1 on the "Billboard" Hot 100 singles chart in December 1971 and January 1972. It reached Number 1 in Canada and Australia and Number 4 in the UK charts.