If you complete your 12th, pass in all subjects and secure a minimum aggregate of 50% in Physics, Chemistry and Biology, then you are eligible for NEET. It is 50% for General Category, 45% for PWD and 40% for OBC,SC,ST. This doesn't matter even if you have secured this through regular exams or supplementary exams. So lets say even if you fail now but pass in Supplementary exam and secure the aggregate for your Category, you are eligible for NEET.