Your sentence is describing a smaller group of a general population, this is usually said to be

some of a population
many of the population

so, the correct sentence is

...many of whose minds are closed...

Your sentences are describing a smaller group of a general population, one way it can be said is

some of a population
many of a population
a few of a population all of a population

another way this can be said is to just mention the quantity directly

many something
some something
a few something
all somethings

Both of your sentences are correct.

Your sentence is describing a smaller group of a general population, this is usually said to be

some of a population
many of the population

so, the correct sentence is

...many of whose minds are closed...

"Many whose minds are closed" is incorrect.

Your sentence is describing a smaller group of a general population, this is usually said to be

some of a population
many of the population

so, the correct sentence is

...many of whose minds are closed...