To determine the quantity of class C networks each of those 32 subnets will contain, you subtract the number of bits you aim to use from the traditional, "classful" mask, and then use the difference to find the binary exponent.
This time, we are interested in (blocks of) traditional class C networks, each of which uses a mask of 24 bits, and we still aim to use a 21-bit mask. So the equation is:
24 - 21 = 3
The binary exponent is 2^3 or 8 class C subnets per block.