I think 20% is high. My uncle was a mission president in Ireland; 17 missionaries left his mission early. If there are 53,000 missionaries world wide a 20% early return rate would mean a whopping 10,600 won't make it! That seems high to me. A ten percent rate would equate to 5,300, which is still a lot of missionaries. Let's say ten percent is the right number. If there are 360 missions, then each mission would see about 15 return earlier than expected in a two year period, which would mean 180 missionaries coming home every month, or 6 in transit every day of the month!
The real number would be interesting to know because it casts more light on a problem that could use a little more attention, in my humble opinion. Could some of these missionaries have been saved? Rather than send them home, is there not some way to keep them in the field in a productive capacity? It's a difficult question to answer; the details for each instance are as unique as the missionaries themselves. More on this later, but we're smart, inspired people. Surely there's an alternative.