i am aiming for 99.7 percentile in 2021 and could you please tell me how many marks should i get out of 300
You have a good aim. 99.7 percentile will definitely gain you the top NITs and IIITs. Now, the "marks vs rank" changes every year depending on the difficulty of the paper, and the number of candidates appearing for it. For scoring this percentile, you need to score 240+ marks. Stay focused throughout the year and solve the sample question papers daily.
Hope this answer helps you.