How much hp would a cold air intake add to a 2012 honda civic?

1.5 liter engine
140 hp

A few, far less than 15 is my guess.

As compared to the cold air intake installed by the factory? 0 to 2

A 2012 Honda Civic comes stock with a cold air intake. The best thing to do is modify it a little to make it a forced air intake- meaning direct some air into the air inlet up-stream the air box from under the bumper.
Aftermarket CAI's are eye candy and nothing more.

It won't. Look where the intake is already… Yep at the front of the car.

Zero… Your car came with a perfectly good CAI from the factory. It's just plastic instead of shiny chrome and has a convenient throw away air filter.
The oil mist on cleanable air filters migrates to the MAF sensor wires and ruins expensive MAF sensors.
The only horsepower increase you'll feel is to install a cat-back exhaust system which increases the power by an average 25 horses.