Andrea,
There is a fixed number of people for each course but this isn't always the same for each course - e.g. April/May seems to be exceptionally large in comparison with other recent start dates. It also depends on how many people failed the course last time and will need a space to re-sit. However, if you pass Stage 3, you will get on a course, but when that will be is another matter...
All people who pass a certain mark on the Stage 1 tests get asked to complete the online personality questionnaire.