So basically an A level is made up of qualifications AS and A2.
First year most students will take between 3 and 5 AS subjects (usually 4) and then will see how they are doing when they get their results and will decide what they want to focus on and will generally choose 3 and do they A2 for those subjects... Basically you can do 3 but its always good to an extra one just in case!
As for home schooling I'm not too sure exactly how it works I'm sure there are some here that have a greater knowledge. i did have a friend who did it and she had to pay for her exams and so on but its possible. Also we had a 9 year old kid who came to our college to take his math tests with us... So I'm guessing you would go to the nearest testing centre just like everyone else (: