I don’t know the answer to your question about how many are in the hold pool.
some people stay in the hold pool longer than others. The people in the hold pool will have been graded from their assessment day and their sim check, they will then be cherry picked from that, as well as their preferences, so if someone put that they only wanted an BHX or EMA base and someone else put any base, the one that had any base would be called prior to the one that was more restrictive if a GLA base came up.