Questions like this are answered by the binomial coefficient. The binomial coefficient
(read "n choose k") means exactly what you're looking for: "How many ways are there to choose k elements from a set of n elements?"
In other words, it counts the number of possible subsets of cardinality k from a set of cardinality n. It is defined as follows:

Where n! is the factorial of n:

So, you have

Of course, a lot of factors simplify:
