I am attempting to create a raffle ticket to purchase form

I need to know how I can have a unique number calculated on each submission which would represent the raffle number on my purchase form. 

Please advise

Thank you

You can add custom unique IDs to each of your submissions. Inside of this guide you can find how to do it:

How to Add Custom Unique IDs to your Form Submissions?

Hope this will help. Let us know if you need further assistance.