Small correction, the insignificant number was 4155 (it was me who posted that thread).
True, so for what it is worth (not much), when I started my guild about a month and a half ago, I seem to remember the start rank was something with 18k.
I have no statistical data, but I can give my impression after working the ranks from 18k up to current 4k.
Very rarely has a guild over 10 members. I would say the huge majority of low-ranked guilds have 1-5 members, a few even 0. Once in a while there is a low-ranked guild with over 20 members.
Around rank 4k most guilds have reached the max member limit (30), but the big majority of guilds still have a single-digit number of members.
I looked at the 50 guilds from rank 4003-4052:
Members - Number of guilds
Only 12 of the 50 guilds have max-member lower than 30.
35 of the 50 guilds have less than 10 members.
Average is 9.09, but the 6 full guilds are pushing that number.
Median is only 4 (27 of the 50 guilds have 4 or less members)