Commas can often be used in very large numbers to make them easier to read and interpret. Commas can be placed every three digits to denote gaps of a thousand. There are three data points that contain numbers that we should not add commas to. You should not add commas to serial numbers because it changes their function. Commas are often left out of reference numbers, you will notice this when you get a confirmation number for a hotel stay or plane ticket. Commas are often left out 4-figure formats for the year (such as 2018).
These worksheets explain how to correctly place commas in large numbers, and how to read the result. All numbers have either five or six digits.