First of all which our subject you are obtained for undergraduation, the eligibility criteria us candidates must have passed their 12th with the minimum qualifying marks. If you got RT in mathematics that means you have to repeat theory.  CBSE provides a chance for those students who got RT in their results for improving their marks by writing compartment exam. It is possible to write compartment exam for one subject. Since you only have RT in one subject you can write compartment exam. If you passed that exam then you are eligible for applying.

You can get admission for BCA either through merit marks in various universities or colleges or through entrance exam conducted by various university's individually or by State governments.

BCA entrance exams include SET, IPU CET, UPSEE,BUMAT,CET, CUET(UG), PASSAT,SPSAT,DSAT, BVET etc. For merit lists admissions, candidates need to score at least 50% marks in Class 12th.

Here is a list of colleges that offers BCA in India refer the article below

https://it.careers360.com/colleges/list-of-bca-colleges-in-india