If you have done your 10+2 from Maharastra Board then you must have applied for the registration and you must have been provided the Registration certificate. As a result you will have the registration certificate in which the registration number is provided. Also in the 12th hall ticket, there also the Hall ticket number as well as the Registration number is provided.

If you still can't find it out then contact your 10+2 school.

To know more about the MAHARASTRA HSC visit the link below:

https://school.careers360.com/exams/maharashtra-hsc-board

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