In 1954, the city of Toronto and 12 surrounding municipalities were combined into a regional government. The postwar economic explosion caused a shift to the suburbs. In 1967, the seven largest municipalities of Metropolitan Toronto were merged into a six-municipality configuration that included Old Toronto, East York, North York, York, Etobicoke, and Scarborough. In 1998, all six municipalities were coalesced into one large city, Toronto, the largest city in Canada.
So, that's why some people, most famously, Drake, call Toronto "the six", due to its previous six municipalities.