How many 9×9 Sudoku puzzles are possible?

How many 9×9 Sudoku puzzles are possible? They discovered that the number of possible 9 by 9 Sudoku grids is N=6670903752021072936960 which is approximately 6.671×1021.

They discovered that the number of possible 9 by 9 Sudoku grids is N=6670903752021072936960 which is approximately 6.671×1021.
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