With a score of 646, your rank might be approximately between 946-1714. This is an expected rank range: https://medicine.careers360.com/articles/neet-result-marks-vs-rank . Now regarding the Colleges that you may have a shot at, you have a chance in:

MAMC

AIIMS, New Delhi

AFMC

AIIMS will conduct its admission through All India Quota but 646 is a decent score. Now AFMC and MAMC details are based on 2019 data and cut offs change every year. Cut offs change every year depending on various factors such as no.of Candidates appearing for the exam, no.of Candidates qualified, top NEET score and difficulty of the exam. For complete list of colleges that you may have a shot at, please use our College predictor: https://medicine.careers360.com/neet-college-predictor?icn=QnA&ici=qna_answer