Hi,

The NTA will mail a copy of your admit card to your registered mail address. So you can check it for knowing your roll number. If that's not possible then you can know your roll number of the exam after logging in to your account using  register number and password and can view your OMR sheet. Your roll number will be displayed in the OMR sheet. If that's not possible then you can mail to authority asking for the roll number. All the best.