As a minimum, students will usually choose four AS-level subjects, one of which will be dropped at A2. General Studies is a compulsory fifth AS or even A2 subject at many schools, but will not usually count towards university admissions. It doesn’t require much extra study, however.
One general piece of advice is: don’t take courses that are too similar. For instance, Biology is very similar to Human Biology. While it’s good to be fairly focused, it’s better to demonstrate a wider breadth of knowledge and skills by picking complementary but different fields, such as Biology and Chemistry.
It’s also worth remembering that exam boards differ. For instance, OCR is reputed to be harder for Maths than AQA or Edexcel. While you probably won’t have any choice in which exam board you use unless you’re self-studying the subject in question, it’s worth looking up which exam board your school uses so you can be sure the kind of assessment you’ll receive will suit you.