I don't think you can track the amount data used on NOW Broadband (unless there is third party a software or an App available that can do it), but if you are worried about using too much data it's unlimited with NOW Broadband.
Just out of curiosity are you talking about NOW Broadband data or data using the NOW streaming TV service and you are with a different internet provider?
Where my above reply is referring to using NOW Broadband.
