You don't add commas to numbers, you add commas to strings when your prepare a number for presentation to the user. And even then, you don't "just add them", as the position of commas as numeric separators depends on the users culture settings. For example:
12,34,567.89 Indian format
1,234,567.89 English and American format
1 234 567,89 Canadian format
1.234.567,89 Italian format
But there are other combinations:
Decimal and Thousands Separators (International Language Environments Guide)[
^]
Basically, you shouldn't assume any particular numbering format: you could confuse the issue quite considerably!