I looked up the code here and it's one alarm in the hall on each floor and one in each bedroom. Houses newer than 1992 have to be wired. Older ones have to be 10 year battery or wired. Since there's easy access I will probably also put one in the attic.

Since we have painters painting the stairway walls and removing the popcorn ceilings throughout the upstairs and will be up there on scaffolding anyway we will have them remove the unreachable one and will replace it with one in the hallway.