If you only study from NCERT book you can easily score around 150 marks in JEE but if you want to score around 250 marks you have to put some extra effort.

To score good marks you should recommended other books as well after NCERT. You can refer previous year papers to know the exam pattern and level of questions. Focus on the important topics you can solve MCQs related to it. Solve as many as mock test series to analyze your preparation at regular interval of time and to know your strength and weakness.

Good Luck!!