Hi Nitesh,

Your Roll no. would be given on your admit card and you will get the roll no once you download the same.

I am providing with the steps to download the admit card

1. Visit the NCERT website, ncert.nic.in.
2. Click on the "Download E-Admit Card for NTSE Examination"
3. Enter the details like the State Roll Number, Date of Birth and captcha in the respective blanks.
4. Click on the "Submit" tab

Hope that helps you

All the Best