Let $S=\left\{n \in N\left(\begin{array}{ll}0 & i \\ 1 & 0\end{array}\right)^n\left(\begin{array}{ll}a & b \\ c & d\end{array}\right)=\left(\begin{array}{ll}a & b \\ c & d\end{array}\right) \forall a b, c, d \in R\right\}$, where $i=\sqrt{-1}$. Then the number of 2-digit numbers in the set S is $\_\_\_\_$ .