It is interesting to compare the numbers of members in the chapters to unaffiliated volunteer contributors outside the chapters. The numbers are taken straight from the page Wikimedia chapters at Meta and the Special:Statistics pages for the individual projects.
The largest member communities belongs to the chapters in Germany with more than 500 members and France with 200 members. The Norwegian chapter has about 55 members and is close to a normal chapter for the moment. Some chapters are really low on the scale, not always because they are in a startup phase but also because there are fierce internal struggles between the chapter and the community at Wikipedia.
If we normalize the number of members to the number of active contributors on the Wikipedia in the most important language, we get some really interesting results. In Norway the relative numbers to the Norwegian (bokmål) version is 2.2%, that is 55 members divided by 2456 active contributors last 30 days for the moment. Similar numbers for Germany is 2%, France is 1.2%, Netherlands is 2%, Italy is 1.9%, Israel is 0.9% and Finland is 0.6%. The Swedish chapter has the very impressive figure of 5.5%.
It seems likely that the numbers will stabilize on a number that is given by a formula including the number of active editors, previous active editors, and the number of readers. Its also very likely that for the near future the number of active editors gives the upper limit, and also is the dominant factor in any estimate for the member base of any given chapter. Probably it is easy to reach the number of very active contributors, those with more than 100 contributions each month, but pushing above number of active contributors would be very hard.
For the Norwegian chapter that would give a member base somewhere between 100 and 2500, which gives a lower bound slightly above the present 55 members.
But turn this around; today the number of unaffiliated volunteer contributors in Norwegian (bokmål) Wikipedia is 97.8%, German Wikipedia has 98%, French Wikipedia has 98.8%, Dutch has 98%, Italian has 98.1%, Hebrew has 99.1% and Finnish has 99.4% and Swedish Wikipedia has 94.5%. Even rather old chapters have a minority role compared to the unaffiliated volunteer contributors.
When chapters speak for the community in those languages they have a democratic problem. They may speak for their members but they can only help the community. Some chapters solve this by diverting to paying and non-paying members, claiming that every member of the community is also a non-paying member of the chapter. This is at least a troublesome claim. Compare this for example with the following from the press contacts page on Wikimedia Foundations own wiki; "The local chapters listed below are independent associations with no legal control of or responsibility for the websites of the Wikimedia Foundation and vice versa. Local chapters and their representatives do not speak in an official capacity on behalf of the Wikimedia Foundation."
So what do we do about this? It seems like this is critical issues that needs to be solved.