In numbers of more than three digits, it's general to use a comma after every third digit from right to left. If you want to add commas into really many number strings, the following regular expression will be useful:
Find: (?<=\d)(?=(?:\d\d\d)+(?!\d))
Replace: ,
The following figure shows how it actually works in the RegexBuddy application.
For more information on the regular expressions, please visit the Reg-Expression.info site (http://www.regular-expressions.info/reference.html.)