How much horsepower do exhaust manifolds add?

Very little. It is better to have duals instead of single as it is harder for the car to breath. But it can add 5-10 horse.

yes it adds about 15 hp depending on the car.

3 to 8 maybey 10 but not likely

5 to 9 horsepower about 7 horsepower i felt a big change of horsepower in mine

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...