stored proc - how to format numbers?

In PostgreSQL (9.x), I have a variable that I want to format with commas.

I do not know how many digits will be in the number, it could be an int
(2Billion+) or a BigInt (18 digits or more).

All I simply want to do is to use:  to_number( and have the commas put
in before display.

There are no example of this in the documentation that I could find.

thanks

J.C.