how much rank is required for getting admission in IIT college. Computer science branch

It depends on which college you want. If you want to pursue Computers in top 7 IITs , then you will have to obtain a rank under 500 to be on a safe side. However if you are ok with new IITs as well, then rank as big as 3000 can help you. In some colleges like ISM Dhanbad, a rank around 4000 may also give you a chance. It would be better if you check opening and closing ranks of various IITs for last few years which will give you a rough estimate. Or have a look at the following link
http://www.askiitians.com/iit-jee/advanced/opening-closing-rank.html

You should score more than 50% marks to get good branch in Top IITs.

How much marks are you expecting in JEE Advanced?