Yes , you can apply and appear in IIT- JEE exams even after repeating class 12. But to get selected in any NITs or IITs you have to score your minimum percentage according to your caste criteria in your class 12 board exams. To get selected in JEE advance , you have to clear the cutoff marks provided in that particular year. For your preference you can see the previous years cutoff marks. Generally cutoff marks increases every year. For example if the cutoff marks in 2018 was 75 for general category , it would be roughly 80 in 2019. Otherwise if  your JEE main marks are good and your 12th percentile is high , you will get a good NIT with a demanding branch. But you can repeat the exams upto a certain time.

Hopes this helps.

All the best!