I have reason to believe that in Canada the ABDL population is at least 1 in 10,000.
I run a ABDL meet up group and have been doing some pretty active promotion of the website over the last year. The membership just reached 120. Out of those 62 of them identify as being from my city of approximately 1.2 million. There are another 6 that just identify as being from the province with no city listed. I don't allow minors on the site because it focuses on real life meetings. From my experience on fetlife, diaperspace and other site I know that there is a significant portion of the population not a member of my site. I feel pretty confident that doubling my number of 62 would not over estimate the AB, DL, and TB population. So 1/10,000 is not an unreasonable expectation.