Because ultrasonic sound doesn't pass through solid objects (like falls and furniture) - the sound won't travel in-between rooms.
So using a few devices will only keep pests away from those areas - but there may be pest activity in other places.
Our previous customers have gotten best results by using 1 device for each room / area in their home.
The more devices you use, the more cornered the pests feel in your home, so the results are far quicker.
Here is a rough guide
1 pack - For a single room or small office
4 pack - For condos and apartments
6 pack - For home with 3 bedrooms
8 pack - For home with 4 or more bedrooms
12 pack - For restaurants & commercial buildings
The # of units is usually the total open unobstructed areas that need protection from rodents.