Hello Aspirant! Studying only NCERT for JEE Mains is okay but not enough. Going through only NCERT books might get you in trouble and lessen your marks too. You need to practice questions and concepts from some high level books like Arihant and other for different subjects. You also need to give and practice as much test papers as you can. Give tests and analyze them at the end. Make short notes and learn from them. These little things might help you boost your marks to the maximum so please try following these tips.
Hope this helps. All the Best!