how to add numbers

Depends on the programming language you are using. In most of the languages a short-hand increment operator is enough. But first step would be to get the variable value. I would assume the variable that holds the current value is indeed, "receipt_no".

The code to increment the value would be then simply,

receipt_no++;

Enough! That is required to increment the value by 1. It is known as post-increment, which increments the value after this expression. In this case pre or post won't make any difference.