Well here’s a pretty interesting site...
List of Billboard Hot 100 number-one singles from 1958 to 1969
https://en.wikipedia.org/wiki/List_of_Billboard_Hot_100_number-one_singles_from_1958_to_1969
15 Motown songs on the list, 12 of them by The Supremes (2nd only to The Beatles). Still only a small percentage of the #1 songs for the decade. If you look at how many LA based artists are in that group, and considering they played on records for everyone from Sinatra to the Byrds and the Monkees I still think the W.C. had to have had more.
You would be wrong.