How many terabytes of data are actually on the internet?

Well Kass, this is really an unanswerable question. As the internet is not a thing in and of itself, and is really the sum of its many parts - it would be nearly impossible to give an accurate measure of how much data is available at any one time.

The internet is constantly expanding, any time a new personal computer, web server or smartphone is connected to and shares data  via the internet, that is technically more data available to any and all other users.