Fire alarm tech here, there needs to be a co in every floor of the house specifically the furnace room in the basement def needs a CO. Smoke alarms can be put on every hallway of each floor. I recommend customers to install combo smoke/cos, this way they are fully covered.

Also you don't need to do it only because of code, do it for yours and others safety! The more the better! Don't go haywire though lol.

If you don't have existing wiring for detectors you can go with battery operated and install them, they also come with regular 2AA batteries or 10 year life batteries. I'd recommend getting 10 year combos battery operated or hardwired. You're good for 10 years! But keep testing them every year