I am currently in 12 and have filled neet form without the 12 board roll no. The correction window's last date is today so is it necessary to add 12 board roll no?

If it is not a mandatory field, you can skip it. But if you haven't utilised the correction window yet, you may as well enter the Roll number in the application form. Entering the roll number makes it obvious that you are appearing for your 12th and then leaves no room for cancellation of your application for providing incomplete information. So please do mention the roll number.