Some help with figuring out where to place access points

We are trying to install wireless devices (Unifi UAR-LR) around a church which is an OLD stone and cinderblock building.  We are up to 5 devices which on its own likely means nothing without the size of the building. But the 6th looks like it should go in an adjacent room to a room we just put number 5.  A cinderblock and sheetrock wall is between these 2 rooms. From the weakness of the signal from one room to the other, we are thinking there is foil backed insulation between the sheet rock and cinderblocks.

Regardless of that, in general, how do you determine how many APs you use and where you put them? Simple trial and error?

We use the insider app on a phone to see how the signal strength looks. What level do you consider acceptable before you need another access point?

Standing directly under an UAR-LR, we get a signal of about -40 to -45 or so. Is that full strength?

I realize that depending on the walls and floors, wifi signals may not travel very far. Does anyone have any rules of thumb? If APs are going to be in each meeting room, do you bother with LR or higher power devices?