Dear Reshma

If I am not wrong, you are asking if everyone should challenge the answer key of NEET2019 or a single person for bonus marks (if found mistake in answer key).

Let me tell you, even if only one person challenges the answer key for particular question(s) and win against it(declared to be right by officials), then he/she will be awarded with bonus mark(s) and the rupees for the challenge will also be refunded to that person. The bonus mark(s) will also given to students who attempted that particular question(s) too.

Best of luck