How many marks I need to score to get admission in IIT

Firstly,i would tell you about the JEE advanced because it creates the main difference.
For general category,around 50 % marks will get you some decent branch .

If you want to take CSE in old IIT’s you should score around 70 % marks.
For electrical,ECE and other top branches,60% will be good enough.
And if you want to get into a decent IIT with a decent branch,you should score atleast 50%.

Suppose if you don’t get a very good rank and you are not getting a good branch in an IIT,you can take whatever the branch you are getting and study hard during the first year of college to get a department change .Department change is a way to change your department based on your outstanding academic performance.

But department change is risky.I would suggest you to study hard during JEE preperation.

I don’t know much about the present system for board marks but even if it’s top 20 percentile,85 % in boards will make you eligible for JEE advanced.And JEE mains is quite easy to qualify . Around 120 marks in mains is sufficient to qualify for advanced.