I know, it's: "Ask not what the NMRA can do for you; but, rather what you can do for the NMRA"! However, I have been seeking numbers on the membership of the NMRA verses the population of Model Railroaders in the U.S. I have been told that membership in the NMRA is around 17,500 people and that this number appears to be shrinking as of late. The number was fairly steady at 20,000 from 2006 to 2012, since 2012, there has been a slow but steady decline.
The population of model railroaders in the U.S. is estimated at around 175,000 people. It would appear that membership in the NMRA is about 10% of the model railroaders in the country. I also know that the NMRA is really an international organization, However, there is no way to bring the number of people not from the U.S. out of the total.
This information was unscientifically gleaned from opinions on the NMRA's Forum. As to it's validity, all I can say is: It is, what it is. Use it; or, not, your call! What is interesting (at least to me) is the 1/10th membership in the NMRA to the overall population of Model Railroaders.
I have found that the NMRA's website is deplorable! I have also found that my regional division, has very little activity and they really don't seem to care about newcomers (It took more than a year for them to acknowledge my membership). I joined the NMRA again, at $75.00 for NMRA and $10.00 for my regional membership. I'm am uncertain, why!