According to the Washington DC-based Pew Research Center, approximately “a third of registered voters in the U.S. (34%) identify as independents, while 33% identify as Democrats and 29% identify as Republicans.” Which would mean for the Cabinet:

Independents- 8.5
Democrats- 8.25
Republicans- 7.25

LINK TO Part I: President-Elect Biden Said His Cabinet Will Reflect The Nation- Part I
U.S. Census Bureau Shows How It Should Look