For Al, its atomic number is 13 and its mass number is 27. How many neutrons does it have?

It necessarily has $14$ $\text{neutrons}$.
The mass of a given atom depends on (i) the number of nuclear protons (and this number determines the identity of the element, because it gives $Z$ by definition); and (ii) the number of neutrons, massive particles of neutral charge. Neutrons and protons are the massive fundamental particles, which determine the mass of a given isotope. And thus for ""^27Al, we know that there are $\text{14 neutrons}$, because if it is an aluminum nucleus it MUST have $\text{13 protons}$.