So transformers can increase or decrease voltage. As with anything in Physics, we often look to see if there is an equation we can use to describe something. In this case we want to know how the numbers of turns on the coils relate to the voltages.
Let's think about things first - with more turns on the secondary than the primary coil, the output voltage (called the secondary voltage) is bigger than the input (primary voltage). Or with fewer turns on the secondary, the output would be smaller.
In fact, if the number of turns on the secondary coil, Ns, is double that of those on the primary, Np, the output voltage, Vs, will be twice Vp, assuming there are no energy losses.