How many questions should i answer correctly in jee mains to get 90 percentiles

For get 90 percentage . You have take 80 to 85 because there is negative marks in jee mains so you have to answer carefully .  So you must answer 35 to 40 questions

For every in correct answer jee cut your -1 so if you correct one answer and incorrect 1 answer it's result is 0 . Rather just confidently answer which you no. So there was a chance or probability to have less incorrect answer.
If your target is 90 percentage then practice a lot.
Study hard, study technically , make a routine , go for mock tests in online platform, study from youtube.
Saw the exam pattern , and also practice previous years question papers. Go for best books

For more information about jee you can visit here https://engineering.careers360.com/exams/jee-main.

Thank you. All the best. I hope this answer helps you.