You’ve seen how numbers are hidden when they are entered in password fields. You can do the same thing in Excel. The number will still be in the cell, but it is shown as the character you define. For example, the custom numeric format,
Displays the letter N for each digit entered. You can still use the content of that cell for calculations, but its display will also be disguised. To see a calculation’s result you will have to apply a normal numeric format.