IIT

Seats vary in every college and depend upon college to college. Please find the details regarding your query in below link
https://www.shiksha.com/architecture-planning/course/b-arch-indian-institute-of-technology-roorkee-8343
I cannot tell you about the all IIT you have to check on your own

Being an SC candidate, you need atleast 65% aggregate marks. It is 75% for general candidate.

Hi Vishnu,
You can use this link to find out your answer. Hope this will help:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor?utm_source=cconnect&utm_medium=CR_JEECP&utm_campaign=6375912.

Both IIT and IIM are different. You cannot appear for IIT exams without maths. Also IIM is for management course which can be taken by the student of any stream.
I am attaching a link for your answer:
https://www.google.co.in/amp/s/www.shiksha.com/mba/articles/amp/20-mba-entrance-exams-you-can-take-this-year-blogId-11473

It depends on your home state. For All of India, I would recommend VIT, SRM, KIIT, ITER, BIT, NSIT and many more. You can go to https://shiksha.com and find great colleges in your home state or any part of the country.
Good luck.

You cannot get seats in IIT on the basis of Jee Mains rank. You have to clear JEE Advanced to get IIT. Seats in NITs are given on the basis of JEE Mains score, but your rank is very low, your chances are less of getting NITs, but you can get colleges of your state. Try JEE college predictor to get more information and insight about college which you can get on the basis of your rank:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor?utm_source=connect&utm_medium=CR_JEECP&utm_campaign=4067795

If you think you can manage the preparations without losing concentration, then go for it. Your class 12th marks suggest that you have the calibre to score 100+ marks in JEE. So, if you can prepare without being frustrated, go for it.

No, you don't get admission in NIT or IIT through JCECE. You have to crack JEE Mains for NIT and JEE Mains and advance for IIT.

You can check here. Use Shiksha JEE College Predictorhttps://www.shiksha.com/b-tech/resources/jee-mains-college-predictor?utm_source=cconnect&utm_medium=CR_JEECP&utm_campaign=7489993.

You need to crack JEE for that.