A-levels are indeed linear, but that does not mean OP's sixth form or college will not make the OP take their AS exams anyway.
I did AS-levels in 2015-2016 (a.k.a. the first year of linear A-levels) and my college made everyone take AS exams. The purpose of this was to see how everyone did in the real exams. Anyone who got less than D in a subject was forced to drop that subject at AS. So, if you get E in maths in the AS exams which do not even count towards your final grade anymore, you are not allowed to take maths to A2.
I know someone who applied for law at Oxford with not four, not five, not six, but SEVEN A-LEVELS. I would say a minimum of four is necessary for Harvard.
That being said, American universities put a lot more emphasis on extracurriculars than British ones, which is worth noting.