A Levels are the final examinations taken by students in the UK system in their Year 13, the second part of Key Stage 5 (the old sixth form). The full A Level consists of AS Level (Year 12) and A Level (Year 13).

There is no maximum number of A Levels that can be taken. Theoretically there is no minimum either, but most universities require a minimum of 3 A Levels, so this is recommended. The most common is to do four AS Levels, and then either continue with four A Levels or drop one and do only three.

A Levels are marked with the following grades (from best to worst) A*, A, B, C, D, E and U. An A Level is passed with grades A,B,C,D or E. U is a fail (Ungraded).
To be able to do an A Level it is not necessary to have done the corresponding AS Level. However, it is recommended because you need at least two years for a complete A Level course (one AS Level and one A Level).

You can register for the A Level directly and then you will have to do the papers that correspond to the AS Level and the A Level. In other words, you take all the exams in the same sitting, as the full A Level consists of AS and A Level. If you have already done a part of the AS Level you can (you have to check the syllabus for that year) save having to do that part (approximately half of it).

A full A Level course may consist of four or six papers, depending on the subject (check the syllabus for that subject).
The exams of the different examining boards are equally valid, in fact it is common in British schools to mix exams from different boards.