A-level courses are designed for the last two years in high school, Years 12 and 13. In fact, there are two different levels one may choose.
AS-Level studies last for one year, Year 12. Like IGCSE and A-Level, the curricula for all the various AS-Level subjects are internationally recognized by all universities.
All subjects taken in AS-Level should be the same as those taken in IGCSE; it's a natural follow-on. For example, if one never took IGCSE Chinese 2nd Language, it would not at all be advisible to jump straight into the AS-Level studies.
If you want to take courses fully on a first-year university level, continue the next and final year of high school, Year 13 with A-Level
These courses are a natural culmination of AS-Level studies and are done in the final year of high school, Year 13.
Because of the rigor of these university-level courses, it is not recommended that one take more than 3 or 4 subjects. One can take as many A-Level subjects as one wants, but it is the truly exceptional student who could handle more than 4 and hope to do well in them.