CO detectors are now required in CA as of July 1, 2011. Check with your local building code department on how may you need. 12 seems like alot unless you have several floors to the house and several bedrooms. Usually, it's at least 1 per floor and 1 in each bedroom. However, it depends on when the house was built too. Again, you can call your local building code department and they can let you know how many you are required to install and where they need to be installed. Also, check with them to see if they can just be battery operated ones or if they have to be hard-wired.