The BSc (Maths) entrance exam is BHU is one of the easiest exams. You can score fairly good marks by just going through your 12th-grade Maths, Physics, and Chemistry books. Study Chemistry from NCERT book. A thorough study of the following chapters will get you good marks:
Biomolecule
Polymer
Animes
Chemistry in everyday life
The questions in the entrance exam are mostly based on direct concepts or formulas.
As far as the number of marks necessary is concerned, there is no fixed digit. But at least attempt 70 questions without mistakes. This means you need to score at least 210 marks out of 450. However, this number keeps changing based on the pattern of the question paper.