To quickly determine the quantity of subnets a given mask will yield, you subtract the number of bits in the traditional, "classful" mask from the mask you aim to use, and then use the difference to find the binary exponent.
In our example, a traditional class B network mask uses 16 bits, whereas we aim to use a 21-bit mask. So the equation is:
21 - 16 = 5
The binary exponent is then 2^5 or 32 available subnets.