HELLO,

If you want to find your 12th registration number , first please check your admit card or official marksheet , as the number is usually printed on these documents and if you still not able to find your registration number then try contacting your school administrative or examination department as they maintain these types of records. If you are from CBSE board, then you can also try finding it on official online portals.

Please confirm your state board so that i can help you with specific steps to find your 12th registration number

Hope it will help !