I'm a general PWD candidate and got 49 marks in JEE this year. Will I get any NIT? What will be my expected rank in PWD category?

It's tough to get an NIT with just 49 marks. Well, keeping the difficulty of JEE 2018 in mind, maybe, you can make it into an NIT but branch may not be good.

Rank usually depends upon the toughness of the paper with 49 marks you stand a chance to get a NIT, chance for an old NIT is less but you may get a seat in new one's. For rank do checkout https://www.shiksha.com/b-tech/resources/jee-main-rank-predictor. All the best.