cani give NSEP individually because my school is not interested in it. how much time do i need to prepare for it and what is the last date of registration, i am in class XI currently

here is no NSEM, for math you have the RMO for which you have to register seperately.

The IAPT website shows that the registration for this year's exams will begin from August 15. So don't worry for now.

Check with your school principal if your school is a center (IAPT might have sent a poster, they did to mine) . If not, don't worry. Find a coaching institute like FIITJEE nearby and register from there (you don't have to be a student at the coaching centre to register)

Finally, if even this doesn't work , wait for IAPT to put up a list of centers. Find a school near you and go register from there. You'll have to pay ₹100 for each.

A word of advice: The Olympiads are hard exams. Pick one or two and focus on them if you wanna do well. Else you might find yourself screwing up all of them.

All the best!