From 2009-2014 (excluding 2010), there were surveys of the LessWrong readership. There were usually about 1,500 responders, which tells you something about the size of the community (note that there are people who read/lurk/comment, but who didn’t submit the survey).
We also have a survey for 2016: http://lesswrong.com/lw/nkw/2016_lesswrong_diaspora_survey_results/