on an average how much should you score in jee mains to get a rank under 2000?

The cutoffs, ranks all these- change every year as these are dependent on various factors like difficulty of paper, no. Of seats available, highest marks obtained, no. Of candidates appeared for the exam. This is also depends on your category which you haven't mentioned. Considering your as a general category student, going by last year's trends, a score within 235 to 250 can fetch you a rank within 2000. However this is subject to change.

More than 250 to get rank less than 2000