As most of you know, there is [no Sudoku problem that has less than 17 starting clues, with one unique solution](https://www.technologyreview.com/s/426554/mathematicians-solve-minimum-sudoku-problem/).

Imagine that there is a sudoku problem that has number of starting clues with one unique solution, where removing any clue in the sudoku will make the puzzle non-unique (more than one answer).

What is the maximum value of possible? Provide an example.