Honestly, the number of your users is directly correlated to your potential market.
I strongly suspect this is not the case, because I have always been limited primarily by "number of prospects I can reach with my marketing" and not "number of elementary schoolteachers in the United States". (Over two million, incidentally. The Bureau of Labor Statistics will tell you all manner of fascinating things!) Nor does the percentage of teachers in the population matter very much at all, because I never address marketing to "the population", I address marketing to e.g. "the people who are searching for [bingo card activity for kids]".
I strongly suspect this is not the case, because I have always been limited primarily by "number of prospects I can reach with my marketing" and not "number of elementary schoolteachers in the United States". (Over two million, incidentally. The Bureau of Labor Statistics will tell you all manner of fascinating things!) Nor does the percentage of teachers in the population matter very much at all, because I never address marketing to "the population", I address marketing to e.g. "the people who are searching for [bingo card activity for kids]".