I want to do NIOS 12 science, I am 28 years of age. I am very good in maths, so what should I do after 12 the science?

Hello Tanay!
It's great to know that you are quite good in Mathematics. As it is something rarely seen.
After completing your (10+2) level with Mathematics from NIOS, you can go for the following :-
1. B.Tech courses (As JEE has no upper age limit and even open school candidates are eligible to appear. But make sure to apply for it within two consecutive years from the year qualifying your Class 12th. Say, if you are appearing for Class 12th in 2020, you can sit for JEE in 2020 and 2021. There are two attempts in one year - first one in January and the second usually in April).
2. BCA followed by MCA.
3. B.Sc. in Mathematics/Economics/ Statistics, followed by M.Sc. and PhD to get a job of Researcher, Mathematician or Professor.
4. Graphic design and Animation.
5. Hospitality/Hotel Management, Architecture.
6. BBA, followed by MBA.
7. BMS or other such courses predominantly for commercial sectors.

With Mathematics as one of your subjects for higher studies, you will be able to explore career options like - Actuary, Cost Estimator, Economist, Engineer, Physicist, Market Researcher, Mathematician, Statistician, Accountant and many more.

Please feel free to ask if you have any more queries.
I hope this answer will help you.