The data published in the Grammy Awards website confirms Michael Jackson and Santana won the most number of awards in one night in 1984 and 2000.
Grammy Awards winning list published on the website shows that Billie has won five Grammys that includes Record of the Year for 'Bad Guy', Album of the Year for 'When We All Fall Asleep, Where Do We Go?', Song of the Year for 'Bad Guy', Best Pop Vocal Album for 'When We All Fall Asleep, Where Do We Go?' and Best New Artist award in the 62nd annual Grammy Awards ceremony which was held on 26 January 2020.
On 28 February 1984, Michale Jackson had become the first artist to win eight Grammys in one night during the 26th Annual Grammy Awards. Santana a rock band won eight awards and tied Jackson's record in 42nd Annual Grammy Awards held on 23 February 2000.
Therefore, Michale Jackson and Santana have the record of winning the most number of Grammys in one night, not Billie Eilish.