How many scholarly articles are written in LaTeX?      

How many people use the typesetting language LaTeX? This is obviously a hard question. However, another way to look at it is to calculate the percentage of published scholarly articles written in LaTeX.
To begin, let's take a look at penetration and adoption of LaTeX across various disciplines, as shown in the Table below, extracted from a paper called "Don't format Manuscripts" Brischoux 2009. The results are not surprising here- LaTeX use is limited to the hard sciences, and especially: Mathematics and Stats, and then Physics and Computer Science.

Then, let's look at the total number of scholarly papers published. Scopus lists 2,253,230 citable documents published in 2014, of which 151,085 in Mathematics, 274,287 in Physics and Astronomy, and 255,916 in Computer Science (calculated using the compare tool on Scopus/SciMago).