Normally, Minesweeper shows the total number of mines around a square, regardless of how many mines you've marked. However, some people might find it useful to see how many mines are left that have not been marked.
Also, some people might find it helpful to color-code the numbers.
Choose Minesweep > Preferences
Choose one of the following from the Numbers pop-up menu:
Total Surrounding Mines: The numbers show the total number of mines around a square, regardless of how many mines you've marked. This is the standard option.
Unmarked Mines: The numbers show the number of mines surrounding the square minus the number of mines marked (correctly or incorrectly). No number will be shown in a square if all of the mines surrounding that square have been marked. If you've marked too many mines surrounding a square, a negative number will appear in the square.
Select the Use Colored Numbers check box if you want the numbers to be color-coded (the default). Clear the check box if you want all numbers to appear black.