Even without a sixth subject, you should be able to get into an IIT if you have at least 75% of your total points in your 12th grade PCM (and English) classes. Your disciplines meet the standards for engineering programs, and IITs usually require a minimum % on the qualifying exam. To be sure, though, always verify the entrance requirements of the particular IIT for any modifications or new requirements.

No there is no issue if you have only 5 subjects they will count your five subjects, where Math's, Physics, Chemistry, and a language subject are compulsory, and the fifth subject will be either Biology or Hindi, depending on which subject you score most in.

HOPE it helps!!