Pretty straight forward. Start with the number of freezers you have. If 2, then you need 3 sensor probes - one for each freezer and the house. Next, determine if your freezers are more than 40 feet apart or 40 feet away from your wireless router. If so, get the wireless gateway.
In my case, 3 freezers so I needed 3 probes/sensors. Irrespective of the distance, I got the wireless gateway, which functions as a wireless repeater so your sensors can be farther away. The gateway essentially communicates with all sensors/probes and then sends it to your wifi. The gateway way also has its own audible and visual alarm to let you know if one of the sensors show an out of range temperature, disconnected signal, or power outage.
It was expensive but the gateway is important if you have multiple freezers.
Here is screen shot of what I see on my phone with using the gateway hub. You can also look at daily, weekly, monthly and yearly temp history.