I had estimated about 200 myself. One sees the same names coming up in whois searches so often.
Judging by the number of tags which aren't registered to domain registrars, the number could be quite a bit higher, then again there are many owned by large companies not really in the domain business but which own enough domains to warrant their own tag.
I suspect most of those who are not acorn members still have a look round the forum now and then anyway.