hi stunter&bunter,
Are you being interviewed as a Direct Entry Pilot (DEP)?
If so there is most definitely a hold pool, however the timescales in the pool are quite variable.
I passed my sim over a couple of months ago and have not had a course date yet. I also know of a couple of other people in a similar position and we are all currently rated on different aircraft.
You will enter the hold pool based on your original interview date. I think BA then try and take people from the pool in order. However, if for instance they had a 757 course and only No1 in the pool and No30 in the pool already had the rating, then it is likely they would get the call rather than just taking No1 and No2 (say Airbus rated).
As far as numbers I have no idea as BA do not usually pass on that information.
A friend of mine passed his sim about 6 months before me and was in the pool for only 1 week, so really it is just luck on the day.
Hope this helps and good luck for the interview and sim.