Thinking about the broker question you brought up, I think you are correct in that they may hold shares for 100 or more shareholders and be reported as a single holder.

The transfer agents likewise control the number of shares but not the shareholder total.

The best information would come from the company as they have to print the annual reports and therefore have a pretty good handle on the approximate number.

I now have a problem with how the SEC allows a company to report false information on the number of shareholders if what we conclude is correct.  Time to check this issue out just to know in the future what is treason behind the reporting.