I got a request from a serious collector about the number of subscribers (members) at various points in the history of the NGS. I remembered that I had a page about that on my original web site which I could not find on my computer. I did find the FAQ's that I developed in a spreadsheet back in the day.
Those stats have been added to my new website: http://jbcarey.net/
Click on items for sale and then click on the FAQ's category.
You will see that the numbers vary somewhat depending on the source of the count. It is, therefore, a work in progress (or subject to debate). Overall interesting, however.