A backlog in your UG degree, especially in an important subject like maths can potentially hinder your admission to IITS for a PG program. The eligibility criteria and specific combinations of subjects accepted by various IITs varies a lot . Some IITs might have different policies regarding backlogs.
A backlog would definitely reduce your chances of getting admission in top tier IITs as they usually select candidates with good academic records.
Conditional Admission: You might be offered conditional admission also wherein you are asked to clear the backlog within a certain period.
Try to clear backlog as soon as possible.Seek your academic advisor or department head for advice.
Solve previous year's question papers and take mock tests to prepare.
Keep updating on the admission criteriaCheck on the official website of IITs from time to time for the latest admission criteria and eligibility.
Keep abreast of changes in the admission process or specific requirements for your program. iit ELIGIBILTY