Hi there

In response to your query for knowing the official roll number of your examination it is highly recommended to go on the official website of the organising authority of NTSE and check out their notices, which would near the examination date; a link for the admit card which probably would require your registration number which is obtained when you conclude the payment option for the application process of the exam. Using the same you would be able to download your hall ticket for the exam which would contain important information one of them being your roll number.

Hope this answer helps; best wishes