how to get a reference number in html email which is sent to customer after form summited

I created a wordpress plugin to calculate medical cost, and send email to customer who use that calculator after click on start saving button. the email is going to client perfectly, but I would like to send him a reference number also in that email and want that same reference number on my email which is collecting the customer information. Simply I want to generate a random code "1 to 9999999" to that email. Please help me