While the NCERT is a must while preparing for your NEET exam, one cannot entirely rely on that. One neither can predict a definitive number of books you need to study for NEET. The NCERT textbooks are to make you familiar with the topic. And, help you have a better understanding of the same.
But, if you want to score well on your examination and not just “crack” you have to refer to other textbooks and online links with the necessary information. NEET is not about how much you mug up. It is about how much you understand and how well you as a candidate can put it across the table.
It is advised you procure as much knowledge regarding the subject through not just one but multiple sources.
When it comes to competitive national level exams, it is advised that you seek guidance from the various study material. And, leave no stones unturned in your preparation.