how many percentile is enough for admission in NIT for obc

For admission in NITs, you need to be among the top percentilers, unless you belong to a reserved category. Few criteria will affect the cutoff score for admission to NIT- Home state quota or all India Quota and your category. All these factors effect the cutoff and it change every time. But in general, try to keep your percentile somewhat near 95.5 to 96 if you are applying through state quota.
http://engineering.careers360.com/articles/nits-cutoff-for-top-btech-branches