What is the Highest High Definition ever?

There are several answers to this question. The highest definition used by broadcasters is 1080i or 720p. Both use the same data rate: 720p refreshes the image at twice the rate of a 1080i but at a lower spacial resolution. There is no expectation that 1080p will be introduced as a transmission format for a few years yet.

Local HD sources such as Bluray can deliver 1080p - twice the data rate of 1080i.

Specialist graphics systems are capable of 2000 and 4000 line resolution but at the time of writing, do not operate in real time. Processing such high resolutions can take minutes or hours for each second of real time video so they are normally used for processing in film or will be converted to one of the HD broadcast standards.

Even as this answer is being written, higher resolutions are no doubt being developed. The limits are based on computer processing speed and storage space. As both increase, so resolutions in post production will increase.