It is difficult to know how many will move to senior from the Junior prep, the school are likely to tell you that nearly all will.
Anything less would make the senior school look less attractive and they still do have to encourage people to take the exam and apply and you may reasonably query if not all the juniors wanted to move up.
Have come across schools saying this but actually some do move one elsewhere- parents opt to move house at that time, other may have decided they can afford Junior fees but not senior (may have thought they could 7 or 8 years before - but they have gone up a lot).
Paying junior fees is not just about getting the child into the senior school - if they don't get a place despite being at the junior prep the school may actually be doing them a favour as they may struggle ... or maybe the school can't tolerate the behaviour any longer
- has been known.