1. 14 - usually sat in two chunks. Typically take 6-12 months to complete all 14.
2. Ones issued by an approved school - e.g. Oxford/ Bristol Groundschool.
3. There aren't any, although approved schools like the two mentioned above receive feedback from the CAA and therefore have a pretty good idea what's in them, thus they can provide you with realistic in-house mock exams.
4. All exams are from one governing body, namely Joint Aviation Authority (the JAA). In the UK, the Civil Aviaition Authority (CAA) administers the exams on behalf of JAA.