Hi there,
Hope you are doing well
18800 rank in JEE Mains is very good and can fetch you decent branches in some of the NITs.
By your preference Mechanical Branch, here are the most preferable NITs you can get a seat in:-
1) NIT Hamirpur
2) NIT Puducherry
3) IIEST Shibpur
4) NIT Raipur
5) BIT Mesra
6) NIT Patna
7) NIT Silchar

Hope this would help.
Thank you

Binit, 

Based upon your rank the best option available for mechanical engineering in nits are

• NIT JAMSHEDPUR
• NIT JALANDHAR
• NIT DURGAPUR
• IIEST SHIBPUR
• NIT HAMIRPUR
• BIT MESRA
• PEC CHANDIGARH
• NIT SILCHAR
• NIT UTTRAKHAND.

If you are just looking from placement perspective then NIT JAMSHEDPUR should be your choice .

But if you want quality education as well as a good education and want to get placed in dream companies like Honda then opt for NIT JALANDHAR. 

If you are ready to pay some extra money then BIT MESRA is the beat option.  Others are good too... But i would suggest you to join BIT MESRA or NIT JALANDHAR. 

You should consider every options yourself before taking admission. 

Hello, hope you are doing well, with your rank of 18800 it is some difficult to get into nits since you are of general category. But i suggest you apply for some nits like NIT Manipur through counselling, there are chances that you can get into the nit since you are choosing mechanical. To know more about the admissions through jee mains, kindly go through the below link

https://engineering.careers360.com/jee-main-college-predictor

I think it helps you!!!

Keep Smiling:)

Wish you good luck!:)