The functional equation \( f(f(x)) = f(x)^{2024} \) implies that \( f(x) \) must be a polynomial of a certain form. Upon solving the functional equation, we find that the solutions are limited to constant polynomials, identity functions, and potentially other special cases. After considering all possibilities, the total number of solutions is 3.
Thus, the number of elements in \( S \) is 3.