The cut off varies year to year as there is nothing declared for Jee main 2018 cut off but as per previous year to get rank under 5000 you will have to get 180 - 200 marks as this would be the safest score but in case if you wish to get seat in top NITs and in top braches like CS,ME, ECE etc then score should be 230 or above.

For reference, you can go through the below cut off of previous year -

Jee main cut off

Hope this helps!!!