Consider the function
$$f(x)=\begin{cases}\dfrac{a(7x-12-x^2)}{b|x^2-7x+12|} & ,\; x<3\\2^{\sin\left(\frac{x-3}{x-[x]}\right)} & ,\; x>3\\b & ,\; x=3\end{cases}$$
where $[x]$ denotes the greatest integer $\le x$. If $S$ denotes the set of all ordered pairs $(a,b)$ such that $f(x)$ is continuous at $x=3$, then the number of elements in $S$ is: