Minimal Sudokus
In a classical 9x9 Sudoku grid you need at least 17 clues to get an unambigous Sudoku. It is not proven, but there is strong evidence for that statement.
If you remove given number from an existing Sudoku which has a unique solution, you may end up with an ambiguous Sudoku, e.g. multiple solutions for that Sudoku exists.
Over 30,000 Sudokus with only 17 given Numbers have been found. I wrote a little script that takes away each number consecutively and checks if the resulting Sudoku has a unique solution. Not one of them had a unique solution.
Therefore you may assume that 17 given numbers is minimal, but you can't be entirely sure that there doesn't exist one with only 16 numbers.
Resources
Gordon Royle has a huge collection of 17 clue Sudokus.
You can play minimal Sudoku download them as pdf on this very site (sudokugarden.de).
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