Hii shivangi!

If you want registration number for class 11th you can check your marksheet or admit card as the registration number is most probably mentioned there. In case it's not present in either of them you can go to your school administration office and ask the office staff for it.

They may ask you reason for the same and if you provide them the genuine reason they may provide you the registration number from your official documents.

Thankyou :)