Assuming that a portion of these hard science papers were written in LaTeX (using the percentages of LaTeX penetration from Table 1) we find that in 2014: 418,732 out of 2,253,230 articles (18%) were written using LaTeX.
The number of LaTeX articles was calculated as follows:
Mathematics and Statistics: 151,085 (92% LaTeX = 138,998)
Physics and Astronomy: 274,287 (60% LaTeX = 164,572)
Computer Science: 255,916 (45% LaTeX = 115,162)
What do you think? Do you have other ideas on how to estimate how many people use LaTeX? Let me know by commenting on this post.