How much horse power does fuel injection give a car?

This would depend on the situation, if you are talking about installing an aftermarket EFI (Electronic Fuel Injection) system on a carbureted engine then the amount of gain can be substantial, although the main reason to install this modification is to prevent loss of power at high RPM.
The main advantage of fuel injection is the fact that it is much more reliable then a carburetor because it has less parts and needs almost no adjustment due to the fact that fuel mixture and injection timing are controlled by the car's computer.
Lack of adjustment is however the main drawback of EFI, most fuel injectors are designed to output a certain amount of fuel per millisecond, therefore the only way to significantly increase the amount of fuel going into the engine is to install larger injectors and a stronger fuel pump. This modification is usually necessary when one is installing a turbocharger or a supercharger.