Only four were selected to train out of all the applicants, to fill the available places. The exact number of places changes each year depending on the projected need, but these places are always heavily oversubscribed. Don't let that put you off, though - if you're good enough, you have a good chance. You need to discuss options with your recruiter.
Edit: The EHT selections take place each year in October at Keogh Barracks. The exact date has yet to be decided but the recruiters will be advised in due course.