Along with qualifying Jee Main, there is another criteria for being eligible for Jee Advanced and that is the candidates must have secured at least 75% aggregate marks in Class 12. 5% relaxation in marks for Sc, St and pwd candidates. Or the candidates should be within top 20 percentile of their respective Class 12 boards. The subjects considered to calculate these marks are - PCM, a language with highest score, another subject other than these four where you have got highest marks.
But this year it's different. As all the subjects of class 12 of several boards have not conducted and marks have been provided on the basis of internal assessment and previous exam marks, many students have affected by these. So this year, the criteria of at least 75% or top 20 percentile has been withdrawn only for this year and all the students who have qualified jee main are eligible for Jee advanced. Those candidates will have to show their 12th passing certificates only. So if you are asking about admission of this year, then you are eligible to get seat in IITs as the 75% criteria has been withdrawn for this year. Admission will be based only on your Jee advanced rank. Read our article http://news.careers360.com/jee-advanced-2020-eligibility-criteria-relaxed-hrd-minister