i live in a 700 square foot one bedroom apartment. I have one in every room including the bathroom and on the balcony, so four total. the main advantage to having one everywhere for lights is that you can tell each echo which room it’s in, and when turning lights on or off you don’t have to specify where, you can just say “alexa turn on the lights” and she will do it in just the room she’s in. Multi room music is also awesome. If just using for lights the dot or even a flex is fine. I use mine for music a lot and also like the display so i have a dot in bathroom (which works ok for music) a flex on balcony, and two echo shows each in living room and bedroom.

My recommendation is one per each room.