When I first left I tried to make sense of the numbers. Very difficult, as the baptized number will be necessarily already be Publishers. As to numbers leaving, one would have to allow for the normal death rate, but this may be slightly higher than the average, as JW's are an ageing lot.
It was patently obvious that even taking those things in to account, there are thousands of "missing" JW's over the decades, some may of course be still attending, but inactive and therefore not counted.
I would hazard a guess, not based upon much, that about 15% leave in any one year, but before that have become inactive.
There even could be millions of ex JW's still alive ???