It is difficult to talk in terms of marks in JEE main, so we will talk in terms of JEE Main All India Rank. For Computer Science branch at NIT Silchar, last year cutoff for EWS category Home state candidate was around 5000. While for Electronics and Communication branch, it was around 9000 and around 13000 for Civil branch at NIT Silchar.

So, scoring ranks under 12000 will definitely fetch you one of the branches you mentioned at NIT Silchar easily.

For more information, you may visit, https://engineering.careers360.com/articles/jee-main-cutoff-for-nit-silchar

To get an idea about marks required, you can check the previous years' marks v/s ranks and get an idea about the same.

Hope it helps.