It really depends on your setup. Is this for fermentation inside a fridge? If so you will need a temperature sensor to measure the air temperature within the fridge and another in a thermowell deep inside the beer. You can have other sensors for logging only.
@Dan yes this is inside a fridge. I was more asking from an actual control point of view, does the program/code control off both sensors or does it just use one sensor to control the temp and just log the other sensor for show and tell?