Yes, you can enter your current OBC-NCL certificate number during JEE Main registration even if it may later be replaced. Final verification happens only during JoSAA counselling, so you can submit a new valid OBC-NCL certificate with a different number at that stage. Just ensure the updated certificate is
You should not leave the OBC-NCL certificate ID blank in the JEE Main form it can create problems later. NTA wants the certificate details while filling the form, not just at counselling. If you can, apply for the OBC-NCL certificate immediately so you get the ID on time.