If you are using 12th admit card as Identity proof then the Identity number will be your Roll Number/Hall ticket number (which ever mentioned on your 10+2 admit card).

I would also recommend you to read the brochure carefully while filing up the form and see if there are any specification provided or not regarding the Identity proof because, in most of the examination the 10th admit card is used for identity, So kindly verify it for your safety.

I hope this helps. Wish you a great eve!