how to find the number of a subset in a set?
Your question is not clearly stated.
Do you want the number of elements in a particular subset of a given set?
or
Do you want the number of subsets possible for a given set?
or ...
e.g. suppose the have a set of 6 elements such as
{A,B,C,D,E,F}
The number of subset is 2^6 or 32
that would be the null set, single element sets, 2 element sets, ... , the entire set
Now consider the subset of 4 elements,
there would be C(6,4) or 6!/(4!2!) which is 15
for a set of n elements, there would be 2^n subsets
of those, the ones that have r elements there would be C(n,r) of them
hope I came close to answering your question.