The number of sensors you need will depend on your goals -- we typically see these two scenarios:
Crisis alerting: If you are only looking to be notified when conditions are not ideal, a lower concentration of devices will be required. Common occurrences include equipment failure (A/C, heater, dehumidifier, exhaust fans, lighting timers) or simply user error (accidental unpluggings, incorrect timer settings, leaving a pump or fan running, etc).
Optimizing microclimates: A higher concentration will be required so you can setup multiple zones within a single growspace. Common placement includes putting a sensor near your best performing tray/plant/bed and another near your worst. This will allow you to compare and understand why one area is performing better/worse than others.