Indian Passport File Number is longer than 12 digits - use last 12 numbers
There is a chance that your existing passport’s FILE NUMBER is longer than 12 digits .
The Indian government online passport form ONLY allows 12 digits to be entered.
In that case, enter the LAST 12 digits of your file number to enter in the online government form.
CKGS guidance for file number longer than 12 digits
This is an official response from CKGS to answer this question:
Let me know if you need more information.