I belong to general open category , my jee mains rank is 21946 and expecting score in jee advanced 2018 is 123,which nit/iit is preferable?

Chnces for NITs look bright as per rankings.

You xan get NITs like Hamirpur, Durgaour, Silchar, Goa, Puducherry, Arunachal pradesh, Mizoram Meghalaya, sikkim and Manipur.

Good chances with IIITs like Sonepat and Nagpur.
You can also get BIT Mesra and Nirma Ahmedabad and Upes Dehradun.

From your JEE advanced scores, there are negligible chances for conversion of IIT as only top 10000ranks have good prospects.

I don't think that you'll get admission into an iit ,so you should focus more on mains counselling.based on your rank.i think you should try a mediocre branch in a mediocre nit.that will be the best choice for you.try going into nit hamirpur.it has a good reputation.