I normally don't rewatch too much so most of the local content will be new 4k content.
If these are videos you are encoding yourself, what codec do you use? What bitrate do you use? What is the source material? (the noisier the source, the less effective the compression)
Encode a 4K film using whatever process you typically use. Take the resulting file size and divide by the runtime (in minutes) of the film. That'll give you a rough per-minute estimate. Take the available free space on a 2TB iPad and divide that by the per-minute estimate to calculate the total number of minutes the iPad can store.
With this final figure in mind, you can then calculate which movies and TV shows can fit. Since there are no standard lengths for tv shows or films, it is not possible to give a blanket, "x" number of films or "y" number of tv episodes.
If the video will be displayed on an external 4K display then it can be helpful to have the video in 4K, but if the video will be viewed on the device itself, then it is unnecessary and will cause excessive battery drain.