I got 40%ile in Jee mains jan. I belong to SC category n can i get admission in nit or any other best college in Mumbai.

Hello.
It is little bit difficult to predict the actual rank so your approximate rank at your mentioned percentile would be: 521,406 let's suppose number of candidate in April exam increased to 12,00000 then you can calculate your approximate rank through the given formula: ((100-your percentile score) X 12,00000/100)
you can predict your chances of  having admission by simply matching your approximate rank in the previous year cutoff through below link,

https://engineering.careers360.com/articles/jee-main-cutoff
Apart from this if you are expecting some more colleges in any city then you should go on the below link where you can predict your college by using career 360 college predictor tool.

Best of luck.