The number of smart detectors you need depends on the size and layout of your home. The National Fire Protection Association recommends installing a smoke alarm "inside each bedroom, outside each sleeping area, and on every level of the home, including the basement." CO alarms have similar requirements, so you don't have to worry about installing extra detectors anywhere you wouldn't normally place a smoke alarm. You're not required to replace every device with a smart version, but note that by mixing smart detectors and non-smart detectors, important functions are lost -- like the ability to alert you to the specific room in which smoke has been detected.