The number of permutations of N objects is $\displaystyle N!$. But say r of the objects are indistinguishable, then the number of distinct permutations become $\displaystyle \frac{N!}{r!}$. Suppose that there are r indistinguishable objects of one kind and s of another. Then the number of distinct permutations is $\displaystyle \frac{N!}{r!s!}$.