Qualifying for IIT admission with Maths as an optional subject

In CBSE mode, there is an option to study Mathematics also along with Biology. physics and chemistry as their group subject. This group is known as the MBiPC group. Are you talking about this group?
The students who pass +2 with these subjects can appear for Engineering entrance as well as medical entrance also. The facility is there only in the CBSE syllabus I think. If you studied this group you can go ahead and apply for IIT JEE preliminary examination as per the procedure given. But to apply for this examination you should procure a minimum of 65% in all the group subjects. Please see the notification given for the entrance test and apply for the same. But you have to show a lot of interest in mathematics and you have to work hard.

IIT authorities would not mind that your Mathematics was the optional paper in class 10+2 stage, they would be more interested to see you how meticulously you have prepared this subject. So instead of thinking unnecessarily in this issue would not help you.
What is to be done from your end is to pay equal attention to all the three subjects namely Physics, Chemistry and Mathematics. First of start your preparation with the text books prescribed for both eleven and twelve and understand the basics of the subjects including Mathematics. If your school is following some other text books, you refer to the text book of CBSE model so that your concepts relating to this subject is sound enough to help you in solving the problems independently.
The later part would be to resort to advanced manuals in Mathematics written by Sri M.L. Khanna, Sri R.D. Sharma or taking guidance from a reputed institution so that you could handle all sorts of questions set in IIT.
You need to chalk out an intelligent planning so that your aggregate at your 10+2 stage does not fall below less than 65 %. For your better scores in the IIT test, appear for the mock tests as many as possible and your impressive scorings in such tests would boost up your confidence and ultimately you would be successful in your venture.

As mentioned above, you can appear in IIT entrance exam so you should focus on preparation of IIT. As you know IIT is reputed Institute and it is not easy to get admission into it. But one important thing, you should have minimum 65% aggregate marks in 10+2,only then you would be eligible for applying in IIT. In CBSE board candidate are allowed to take physics, chemistry, biology along with mathematics. Before applying you should go through advertisement and then apply for the same.

With effect from the session year 2021-22, the All India Council for Technical Education (AICTE) has made Mathematics and Physics optional in Class 12 even for the students who want to get admission to B.E. and B. Tech courses. So as the eligibility criterion is changed, one can apply for the competitive examinations or admissions leading to admission in CSE stream.
The students doing their class XI and XII will now have an option of choosing any three subjects out of Mathematics, Physics, Chemistry, Biology, Computer Science, Electronics, IT, Biotechnology, Information Practices, Technical Vocational Subject, Agriculture, Engineering Graphics, Business Studies, and Entrepreneurship.
One thing that a student should keep in mind in this changed scenario is that when there is an entrance exam for engineering in IITs or any other engineering institute through JEE mode one will have to perform well in that and from that point of view selection of subjects in class XII is also a crucial factor and accordingly one has to opt the subjects and prepare for the class XII as well as that particular engineering entrance exam including of course the prestigious IITs.