No idea on how many they say should re-apply in 12 months.
Apparently, the group I was with were told 95% of re-applicants get through the selection again...but don't take this as fact unless they say so.
Best thing to do is be positive and the best advice I could give you is to follow their feedback to the letter! It will most probably be blunt...very blunt but it's for your own good. I was in the same situation, I applied at 18. They said come back in 12 months. I spent the year improving aspects I was told were weak and went back. No the second time wasn't perfect but it was far better and at the end of the day, nobody is perfect and I passed at 19 instead.
It seems that the majority of cadets that get through have a degree, certainly on my course 85% have a degree and under 21s are the minority!
I would disagree with djfingerscrossed though. I wouldn't suggest you get a degree for the sake of it. If you want to go to university to study something you really enjoy then go for it. However, I found that I had so much more freedom and versatility to have the world at my feet and focus on the goal at hand rather than (in my opinion) follow the sheep as it seems the logical and most common thing to do.