I have one smoke detector, located within spitting distance of the center of the house. This is sufficient, no matter what The Experts say:
I know the new “thing” is to push people to have smoke detectors in every room of the house. I will never do that. I live in a small house. I have one detector in the main hallway, which is centrally located and is 20 feet or less from every other room in the house. And I have one (which I really should replace, it’s more than 10 years old) in my bedroom. But it seems foolish to me to have one in the bathroom, which is three steps away from the main detector, and my experience with a smoke detector in the kitchen means you get a ton of “false positives” when you grill anything or broil anything. (If my house was more than one story, I’d have at least one on each story, but it isn’t.)
Two words: alpha particles. Admittedly, it’s not a whole lot of radiation, unless you actually swallow it, in which case you have issues beyond smoke detection, and besides, the half-life is a mere 432 years.