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There is really no way to measure this; all schools are different, and each one would have to keep records of all students who reached the black belt level, and forward that information to a central source.
My own experience is that the percentage of students who start and eventually become black belts is not a high percentage; to do this art correctly you have to devote time and energy to the discipline; you really have to want to do it. Since it can take 3 to 4 years to reach that level a lot of students drop out during the way.
There is no way to know this, since most schools do not keep statistics of this nature and even if they did there is no central recording facility to keep track of it.