To qualify for JEE Mains, there is no minimum aggregate criteria. But if you want to join NITs, IIITs or CFTIs then you should score a minimum aggregate of 75% only in your 12th. 11th marks aren't considered. And then to appear for JEE Advanced which is the gateway for admission to IITs, you will need to fulfill the 75% aggregate condition. Now it is 75% for General Category and 65% for Reserved Categories. So how much you score in 12th is what matters to secure your eligibility for NITs, IIITs, CFTIs and JEE Advanced. You can appear for improvement exams and be eligible for JEE Mains and Advanced. So you needn't worry about it. If your goal is NITs, IIITs,  CFTIs and IITs then you need to score 75%. Or else 69% is good enough for appearing for JEE Mains. But yes for JEE Advanced, you have to appear for Improvement exams. Now JEE Mains is held twice in a year, once in January and again in July. If you are planning on appearing for these 2 exams in 2021, for now stick to NCERT books as the 2021 syllabus is going to be reduced. Either 75% in your 12th or top 20 percentile in your Boards, one of them is considered for checking the eligibility criteria for JEE Advanced.