Sometimes it is just easier to use a little figuring out beforehand and then bruteforce the result.
Think about this, find out how many digits would be at the very left of your number, e.g.:
1,234 has one
12,345 has two
123,456 has three
Okay, now you have that. Now, say for example you have two digits on the end.
Write out the first two digits of your number and then add a comma. Something like,
cout << Number.substr (0,2);
Then output a comma,
cout << ",";
and then output three more digits and a comma, and continue doing that until you reach the end of the string. You don't even need to know how many digits are actually in your string to begin with, only how many are on the very left.