How many appliances can Powersensor monitor?

Powersensor allows you to easily break down your bill by appliance all within the Powersensor App. 

With the Powersensor solution, you can monitor up to nine (9) appliances in total, made up of either hard-wired appliance-sensors or WiFi-Plugs.

You can view how to install an additional plug here

How to Install your Hardwired Appliance Monitoring sensor

Why would I need to buy an additional plug?