When you use the TO_CHAR() function to format a number in Oracle, you use a format model to determine how the number should be formatted.
For example, you could format a number like 12,345.00 or like 12.345,00, depending on your locale.
The format model can include the G or D format elements to add a comma to a number. Which one you use depends on whether you want the comma as a thousands separator, or as a decimal character.
Alternatively, you can use an actual comma character (,) if you prefer, although this method is not locale aware like the G and D format elements are.