What are the Sudoku rules?
Place numbers 1 through 9 on the Sudoku board without repeating a number in any row, column or box.
How many Sudoku possibilities are there?
There are 6,670,903,752,021,072,936,960 unique solvable Sudoku games. No one would ever be able to do every one of them.
Is it OK to guess when placing numbers?
No, it is not at all a good idea to guess. If the wrong number were placed in a cell it would "mess up" the game and you would end up getting very confused. Always make sure a value you're ready to place is correct before placing it.
It seems very natural in Sudoku to only place numbers when you know that that number is correct and goes in a given cell. However there are times, especially with difficult puzzles, when you're not sure what to do next and you may be tempted to place a number to "just try it." There is a technique called "trial and error" or "what-if" where you "try" a number or numbers but that is a different situation.