Some numbers regarding classical Sudoku (9x9 grid):
81: number of cells in a Sudoku grid.
17: the minimum number of givens in order to have a uniquely completable grid.
49151: the number of different grids with 17 givens actually known.
6,670,903,752,021,072,936,960: ~6.67×1021, number of possible ways of filling in a Sudoku grid.