It depends on the motor and the amount of boost. Normal atmospheric pressure is 14.7 psi, so that's the condition your car is running in while naturally aspirated. If you add a turbo that provides 14.7 psi, for example, in theory you have doubled the power output of your engine. That won't actually be the case exactly because of losses to friction, heat etc....
For example.. if your car has 100hp and you add a turbo that provides 14.7 psi of boost, your car will now produce 200hp, theoretically. If your turbo puts out about 7psi (half atmospheric pressure), you will get a gain of hp totaling half of what your engine made normally, 150hp total in this case.