No idea on the number but it is certainly dreadful. Tell my story of woe on here enough to bore everyone to tears but I was one of only three adults in a group of 50 new recruits in my course. The majority of people on there were young footballers of GCSE age or below who were obviously solely there as they could do it for there Duke of Edinburgh award. For those out of the UK, that's an outdoor pursuit award lots of kids do in this country, which used to be confined to camping in the woods etc but has become broader to keep it going.
Most of the people there only wanted to play football during the course and saw the officiating part as an annoyance. The tutors assigned most of the people there to the local youth league to get games and I heard nothing and had to go to another county to get games in the adult Sunday league so I could qualify as they were prioritising these kids, almost all of which were never going to become referees.
4 weeks later I have that league secretary ringing me as he's short of refs. During our discussion it became apparent that almost all of his new refs had left in the first 4 weeks and he was shorter than ever. That probably gives you a decent guide as to why retention is so poor. Alot of referee students actually have no intention of becoming referees. The FA shouldn't allow DoE students to do courses (I assume it's for income that they do). I'd young people want to become refs then target junior clubs who have excess players, particularly those in the bottom leagues.
When I did the Futsal course it was exactly the same. 10 of us there. 4 of which were put through by their club for free. All good footballers, all with terrible attitudes and absolute 0 intention of refereeing. And one chap who clearly did want to be a referee who I'd describe as a bit quirky, but that's another story