This is actually an interesting question. Data:
288 Hunter Exams to date. Over 10,000 people try out each time and get filtered out bit by bit, usually by not even being able to find the testing site.
5 hunters have died in the last year. That is "unusually high" and is being investigated as a likely conspiracy. Everyone thinks the vice-chairman is responsible. I'm doing that from memory of the episode, so feel free to correct this.
The chairman was the 12th Chairman and about 100. We'll say, barring accident or injury, most hunters live to be around 110, because of the healing effects of regular nen use (referenced by a couple of characters).
So if five dead hunters a year is really rare, at least that die in the course of their duties, lets say two per year is average. Counting old age. 288 years, so you've had 576 Hunters die. That gives you an average pass rate in the Hunter exam of 4.3 people. Considering we know for a fact that some years have only had one person pass and other years there are as many as 9-10, that seems close.
I made some broad, though supported, assumptions there and got reasonably close. Numbers seem pretty believable.