For each value of \( c \), we analyze the equation \( f(x) = c \). The number of real solutions to this equation for each \( c \) represents the number of elements in \( S(c) \). After solving the cubic equation for \( c = -7 \), \( c = -5 \), and \( c = 3 \), we find that there are 5 solutions in total.
Thus, the correct answer is 5.