I just counted, I have 10. I thought it was 11, and I rounded up. Anyway, the exact math wasn't my point, but that lithium primary batteries are very expensive. Ironically, I just got a price alert that they're down to $46.64, except that when I try to buy them, Amazon says "Sorry, this item is no longer available from the seller you selected".
I don't have a huge house, but it is a 1950s split-level, and is pretty much the opposite of a modern open-plan design. And yes, I'm careful. Following NFPA recommendations, I have a smoke/CO detector in every bedroom, outside every sleeping area, and on every level, which adds up. Better safe than sorry.
Nest smoke detectors take 6 batteries each https://support.google.com/googlenest/answer/9297548?hl=en#:....
72 batteries / 6 batteries/device = approximately 12 smoke detectors (could be 9, 10, or 11)
We have 6 First Alert smoke detectors that are connected together but don't use WiFi. There's 1 in each bedroom and a couple left over for key places.
Using 12 smoke detectors is being very careful to cover every space or a big house. Or both.