Validity of Ampere circuital law

The law you state is valid for any closed loop (in vacuum and for static electric fields) where $I$ is then the total current running through the loop. However, the expression is only convenient to use, if you have symmetries that allow you to carry out the integral. Therefore, in many text books you only see it applied for planar loops.

The classic example where this law is convenient is the magnetic field around a straight wire. In that case, you place your loop in a planar circle a distance $r$ from the wire and you argue by symmetry that the magnetic field must be the same on the entire loop and hence,

$$oint mathbf{B}cdot dmathbf{l} = 2pi r B $$.

You then arrive at the well-known expression $B = frac{mu_0 I}{2pi r}$ for the magnetic field around a straight wire. It is emphasized that the law you state is still valid for any strange loop around the wire (and not around the wire), but the integral then becomes very difficult to evaluate.