Hello student registration number is mentioned on Class 11 registration card which is provided by CBSE after receiving registration for class 11  from schools along with details of subjects.

You must have received your registration card from your school in class 11 if you've not received it you need to visit your school and collect it.

In case if your school deny then you can visit your regional CBSE office.

Feel free to comment if you've any doubt

Good luck