How can I generate sequential numbering

i want to create aa application form and when downloaded by the applicant the application number has to get generated in a sequential way which should be printed in the application form of the applicant

You can use the Unique ID Widget. The number will auto-increment on every submissions.

Guide: http://www.jotform.com/help/79-Add-Custom-Unique-IDs-to-your-Form-Submissions