Is there any valid, publicly released data on the teams and their membership numbers?
It's one of the things I absolutely love about German football, the 50+1 rule, and while you have a few teams that skate around it, it's still a good rule and one that should be admired.
I did find this chart, but I'm not sure how accurate it is.
https://www.statista.com/statistics/595935/german-bundesliga-clubs-member-numbers/