Well i have no clue but
From the HRC website "Formed in 1984, the HRC boasts active membership of 8,500+ with 132 Clubs in 36 States and 2 Provinces."
EE has 68 AKC hunt tests for July - Sep or about 130 tests as most are 2 day tests. That would mean about 520 tests per year. If we use 90 dogs per test (60 master, 15 senior, 15 junior) and 2 dogs per handler average that is 45 handlers per test. 45*520 = 23,400. If we assume each handler attendes an Avg of 5 tests per year we get to 4,680 handlers.
I am not familiar enough with field trials to guestimate.
So my guess is it is between 10k and 20k. Definitely not determined by an exact science.
I am just glad to be a part of it no matter how big or small it is.