jmc

The given congruence has a solution if and only if $a$ is invertible modulo $12$ since the congruence implies that $a,x$ are inverses of each other modulo $12$. In other words, $\gcd (12,a)=1$. The only such positive $a$ less than $12$ are $1,5,7,11$. So the number of possible values of $a$ is $\boxed{4}$.