The Closing rank for Computer science vary in different IITs. According to the data of 2015 the closing rank for computer science in IIT Bombay from general category was 59. The closing rank of IIT Delhi,Madras,Kanpur and Kharagpur were 102,186,207 and 300 respectively.
The closing rank for CSE in IIT Tirupati and Palakkad were 4796 and 5272.
If you aim to get CSE in old IITs you should aim for rank under 1000.
As per the cutoffs of previous years, it can be concluded that CSE is the first choice of almost every student. So, the cutoff is very high and one have to score a rank below 1000 to get into the top 10 IITs.
If you aim for IIT Bombay or IIT Delhi then you have to be in the top 100 rank.