What should be my intermediate score to be eligible for IITs?

75% aggregate for general and 65% aggregate for SC, ST, pwd category is required. ,

You need to score a minimum of 75% in HSC.

You need to secure 75% to get into IIT or NIT. But practically, it doesn't matter at all. If you can clear JEE Mains with a good score that means you have a great mind and interest. That's what matters the most.