I keep seeing this number, and I think it is quite a high estimate. Scanning the discogs entries for the past six or seven essentials (at least the ones I have), the number of these in people's collections range from 1849 (Caroline Rose) to 6051 (Queens of the Stone Age). The average is more like 3500. I know not everyone uses discogs, so a bit of extrapolation with some assumptions can be made from limited release numbers.
Here are some "In collection"/"total press" numbers for known quantity limited editions:
431/750 = 57%
574/1500 = 38%
342/750 = 45%
592/1500 = 39%
266/500 = 53%
234/500 = 46%
As you can see these vary a bit from 38% to 57%. So taking this range for QotSA, the range of essentials subscribers would potentially be 10615 to 15923.
High end math if only 38% people have in collection: 6051/0.38 = 15923
Low end math if 57% have in their collection: 6051/0.57 = 10615
Another way to look at it, is if there are indeed 30,000 essentials subscribers, that means only 20% of the QotSA collectors are using discogs.
Granted, this makes some assumptions that people are using discogs truthfully. Also, the limited editions are indeed limited to the number advertised. These estimates may even be a bit high, since limited editions attract more collectors who may be inclined to use discogs more than the average user.
I'm sure this calculation flawed is somehow, but I think their subscriber base for one track is closer to a max of 15,000, and averaging closer to 7000.
That's what I do with having more time....