You would not need more sensors than vents, but its certainly an option. The more sensors a home has, the more Ecovent can understand and learn about it. We recommend at least one sensor for every room that's centrally heated or cooled.
In open space, climates can vary significantly, so we recommend you use a sensor every 20 feet to make a different zone. You can plug in sensors anywhere in your home without pairing it to a vent. The sensor will report the temperature, pressure, and humidity of that location and you will be able to see the temperature data in the Ecovent app.