Hello @Harender_Singh_Negi and @Rohit_Bansal
As per information brochure, if "yes" is selected in "Whether appearing for improvement examination of class 12th in all subjects" then Roll Number of improvement Examination (if allotted) has to be entered.
We will try to get in touch with CBSE / JEE Main helpline and try to sort this out. Waiting for roll number does not seem to be a solution as roll numbers will probably be allotted much later when form filling dates get over.
As soon as we get a response from CBSE / JEE Main, I will inform you guys here. In the meantime you may also try to contact officials.