I don’t think that the number of members is listed anywhere, but according to the awards page, 13,966 users have answered at least one question. However, some users have multiple accounts, and some users are no longer active, so that number is not 100% accurate. Maybe one of the founders has a better estimate of how many users are currently active.
I track the awards page. Here is a graph on the number of people getting the Jelly-Asker and Jelly-Helper award over the last 20 days. It doesn’t show the total number of users but you get a good idea of growth.
And I track all the awards If anyone wants the Numbers/Excel file let me know.