There are 16 IITs now in India. There are two examinations JEE mains and JEE advanced. Further, the class XII exam results are also important.
In 12th class one needs to get 80% percentile or more among the students in the respective board.
In the JEE mains one has to get with in the first 1,50,000 students at all India level.
I believe there are around 6000 seats in all IITs together in the open general category. If we allow for those who will refuse their seats due to their own preferences elsewhere, a seat in IIT will be available for a rank of 6,400 or so. This number varies every year.
If one belongs to ST, SC, BC, or physically handicapped categories, then the rank that is needed will be quite different. The closing rank depends on the category one belongs to. The IIT web sites give you the list of last ranks in their institute.