Hi

Class 11th roll no will be the roll no you have in your class 11th marksheet. But if for any reason you don't have it skip filling those details just fill in the basic details like college name or school board or passing year , even if you don't fill it you can still fill the rest of the form and finish submitting it but it is suggested to fill in as much details as you possibly can. Since this is a recurrent problem among many students this year you might hear from the NEET officials soon. But as of now you can contact your college and get the details, but if you can't then as a last resort leave those blank or wait till you get any updates from the officials as the last date is 10th October but don't delay too much. Those details are not mandatory. Decision is yours.

Make sure you fill all the other details rather carefully as it's one time chance you won't get a correction window later.

Hope this helped!