As I understand it at the moment most young people going on the A Levels start with 4 AS levels then carry their best 3 on to A Level (with some exception but in general).
My DS is in Y10 so I don't think this will be an option for him so I'm not sure how to advise him and wondered what others were thinking of doing. He could start with 4 and see if he could carry all 4 through (assuming timetable space) or start with 3 or do 3 plus a planned AS or other smaller qualification.
I'm not sure if it makes a difference that Maths and Further Maths are in his list of A Levels he'd like to do.