JOSSA 2018 admission is over now. JEE Advance Exam pattern & Marks changes every year so it is difficult to predict exact about the score but try to score more than 50% marks to get admission in IIT Madras. You can also check the previous year cutoff rank of JEE Advance with the given below link.

https://engineering.careers360.com/articles/jee-advanced-cutoff-for-iit-madras

Good Luck!