Good afternoon, i understand your issue here's are some way to find out

Ask your school

Your school will have records of all students’ roll numbers for that year.

They can give you the exact roll number or even provide a duplicate marksheet.

Use your board’s official website

Many education boards (CBSE, ICSE, State Boards) have a roll number finder / result archive page.

Usually, you can search with your name, father’s name, mother’s name, and date of birth to retrieve your roll number.