The formula is 2number of elements in the set

This set {1,2,3} has 3 elements, so the number of subsets is gotten by
substituting 3 for the number of elements in the set:

2number of elements in the set = 23 = 2x2x2 = 8

So there are 8 subsets.  They are

1.  { } or ∅,  the empty set, sometimes called the "null" set.
2.  {1}
3.  {2}
4.  {3}
5.  {1,2}
6.  {1,3}
7.  {2,3}
8.  {1,2,3} 

The last one is the entire set, which is considered to be a subset
of itself, although it is not considered to be a PROPER subset.

Edwin