Science

No, its not compulsory but it is better if you add chemistry in the best 3.

Jey Keshav, Apart from curriculum, DU is known for high cutoffs, Fest and for College societies. Delhi University has very few colleges which offers (Hons. ). Biochemistry. The colleges are :-
1. Sri Venkateswara College
2. Daulat Ram College
3. Shivaji College
4. Institute of Home economics
5. Deshbandhu College
6. Shaheed Rajguru College of Applied Sciences for Women
Out of these I would highly recommend Sri Venkateswara College. Besides, DRC and IHE are average choices along with the others.

If you like physics, then I will recommend to take B.Sc. There is nothing special about B.Tech. B.Sc. Is also a good field.

Follow the Andhra Pradesh EAMCET predictor below for details :
Andhra Pradesh EAMCET: https://www.shiksha.com/b-tech/resources/ap-eamcet-college-predictor.

The chances of getting into N. College, Patna University - B.Sc.(Hons.) in Mathematics with the general category are very less.

This is up to the discretion of an institute. Nobody can be sure of the second list.

Yes, the admission forms are out. So, you may take admission.

The chances of getting into the college are good as you belong to BC category. But you still have to check with the college as every year the scores change.

Can you please be specific about what exactly do you want to do in future so that I can guide you better?