How many past papers should i do before the exam?

I cannot stress this answer enough, but, ALL of them. TWICE, at least. When it comes to Maths, past papers will without a doubt be the key to unlock your target grade. Success in maths comes with practice, and anyone who has achieved what they wanted in Maths will tell you that. The more past papers you do, the more adjusted you become with the style of questions, and the easier it becomes to answer them. Also the more often you repeat the same paper, the more aquainted you will become with the methods needed for solutions, and the quicker your timing will become. I would aslo suggest to utilise as many maths resources as possible. For example many online websites may provide their own versions of exam style questions such as PhysicsandMathsTutor.com, and others such as ExamSolutions.com have video explanations of official exam questions from the past. Following these tips should surely get you the grade you desire.