Unique Puzzle Availability
Because the rules prevent so many repeated digits, you might think (as I did) that there is probably a relatively small number of unique Sudoku puzzles that could exist, right? Not true.
In fact, there are over 6 billion trillion different valid puzzle combinations (or "solutions") in standard Sudoku (OK, math was used to calculate that).
And with each of these 6 billion trillion solutions, there are many, many different starting puzzles "hidden" inside.
Suffice it to say that we will never run out of new, unique puzzles — of any difficulty level!