*(A Level = Advanced GCE Exam)
In the UK between the ages of 16-18 we take A-Level exams, where you sit three or four subjects to study for two years, with nothing else. You can take more but since entry requirements for pretty much every university course ask for <= 360 UCAS points and an A grade at A-Level is worth 120, taking more than four is overkill and likely to stress you out.
Since we have around 6 or so exam boards, the syllabus changes among these and certain topics are either not covered or replaced with something else between them. There are also options of taking a "Further Maths" A-Level and even extended A-Level papers for top students (AEA = top 10%, STEP = Top 2%) However, these are aimed only at students wishing to take a mathematics course at University.
On the subject of standard A-Level Maths, the course is usually split into 6 or so units to be studied over the two-year period. On the Edexcel exam board, units C1, C2 and a mechanics/statistics/further pure/decision paper are sat, totalling 3 units. (I'm not too sure about the choices available so don't quote me on it but I'm pretty sure that's right.) In the second year students take C3 and C4 along with one of the aforementioned choices (there are several levels of difficulty in each of the choices, allowing students to take a range of papers as "applied mathematics" where they could theoretically take 5 Mechanics papers of ranging difficulty with a Further Pure paper for another A-Level. It is possible to just study mathematics at A-Level with no other subjects for example taking Maths, Further Maths and Applied Maths.
The papers vary in difficulty and are either calculator or non-calculator. Example papers of the core units of the standard Maths A-Level:C1