How to automatically generate a ticket number and send it in thank you email to the users?

If I understand your requirement correctly, you want to generate a number automatically and send it in auto-responder email to your users. Is that correct?

Yes it is possible to achieve your requirement by adding "Unique ID" widget in your ticket template. It will automatically generate a unique number every time form is submitted. You can add this field in notification email or thank you message and that should solve your problem. You may like to take a look at the following guide which should help you:  https://www.jotform.com/help/79-How-to-Add-Custom-Unique-IDs-to-your-Form-Submissions 

Hope this helps.

Do try it out and get back to us if you have any questions.