Cellular data

Go to Settings/General/About will show you storage.

If you mean how much cellular data your phone has used for a given time period that information is available on your carrier’s website. Also, in Settings/Cellular under Cellular Data the first entry shows how much data your phone has used since you last reset statistics (at the bottom of the listing).

If you mean how much data is available in your data plan you need to ask your carrier. You can probably also find it by logging in to the the carrier’s account management site.

Settings/General/iPhone Storage