how many marks a general category candidate should score in JEE advanced to get admission in top IITs

I can tell you about the ranks, because marks and cutoff's tend to change every year, decreasing the predictability. Getting a decent IIT is not a difficult task if you're ready to put in the right amount of time and hard work. Coming to my point, a rank under 2000 can land you in one of the best IIT's. But if you want to pursue computer science or electrical engineer, you need to have a rank under 1000.

JEE Advanced maximum are not defined :

Maximum Marks vary every year like below:

2017- 366
2016- 372
2015 - 504
2014 - 360

So, it is difficult to predict marks as exam pattern and marks change every year but you should score more than 50% marks to get good branch in Top IITs.