I got 13675 rank in obc category in mains exams which course and which nit will I get
I am sorry but an OBC rank of 13,675 will not fetch you a seat in any of the NITs/IIITs as far as I am concerned. You can try for GFTIs but that also seems unlikely in my opinion.
You can try for some other counsellings apart from JOSAA as well and try your luck there but I don't think you should depend on JOSAA for admission this year.