Answer:
Nine ABBA singles peaked at number one in the UK:
Waterloo (1974)
Mamma Mia (1975)
Fernando (1975)
Dancing Queen (1976)
Knowing Me, Knowing You (1977)
The Name of the Game (1977)
Take a Chance on Me (1978)
The Winner Takes it All (1980)
Super Trouper (1980)
There are some who believe there was a tenth, One of Us, in 1981. It peaked at number three in December, but there was no chart the following week because of the Christmas holidays. The song did reach #1 on some regional charts.
Five of ABBA's eight studio albums also peaked at number one in the UK as did four compilation albums. In fact one of those, ABBA Gold, reached #1 five separate times during the almost 500 weeks it has spent on the British top 100 chart. It is the third best selling album in British history and shares the record for the number of weeks on the chart with Queen's Greatest Hits.