St. Xavier's College Ranchi and Gossner College Ranchi are both great options for a Maths Hons degree. You should consider applying to these colleges.
I'm not familiar with the colleges in Ranchi, so I can't help with that. In Delhi University, though, Hindu College, Acharya Narendra Dev College, and Miranda House are top choices for Maths Hons. Sorry I couldn't be more helpful about Ranchi!