The 12th roll number is the number which is mentioned in the hall ticket of HSC apart from Registration number. In many boards it is also called the Seat Number. So to get this number you need to have the admit card/hall ticket of HSC and you have to put that number in NEET 2021 form.

To know more about NEET visit the link provided below:

https://medicine.careers360.com/exams/neet

I hope this answer helps. All the very best for your future endeavors!