Consider two sets $A=\{2,3,5,7,11,13\}$ and $B=\{1,8,27\}$ . Let $f$ be a function from $A$ to $B$ such that for every element $b$ in $B$ , there is at least one element $a$ in $A$ such that $f(a)=b$ . Then, the total number of such functions $f$ is