How can I see how much data usage my device had?

Go to Settings, go to Network & Internet, tap on Mobile network. Tap on Data usage. There you will see a graph showing what your device estimates its data usage was. Below the graph you will see the amount used by every app, from highest to lowest. Important: Data usage shown on your device may not be the same as the ammount measured by your mobile provider. Keep into account that the value shown there is an estimate.