There's maths and then there's further maths.
Some sixth forms teach maths A level in year 12, then you sit the exam at the end of year 12 and start on further maths in year 13.
Other sixth forms teach the two together in 1.5 of teaching time (so each A level only get 0.75 contact time). Not wonderful, but it's a valid response to squeezed budgets and difficulty in finding teachers who know enough to teach A level maths.
Some schools say that if you want to do FM, then you have to do 4 A levels, because some uni courses say that you can only count one of Maths and FM in your application. However, uni maths courses insist on FM and then they just want one other A level, and they really could not care less about the 4th.
So, how many you do depends on what you want to do at uni and whether you want to retain the flexibility to decide maths isn't for you after all.
A good rule of thumb is if you aren't good at algebra (i.e. if you don't find it easy) then don't look to do Maths A level and definitely don't try to do FM. A lot of people get 8s in Math GCSE without really getting to grips with Algebra, and then they struggle in year 12. So look beyond the grade.
Many sixth forms start on 4 A levels, irrespective of whether FM is in the mix and then drop to 3.