In trying to find the original sources for my table on the number of traditional European windmills I’ve come across some references that should be noted.
Robert Gasch in his book reported that there were 18,362 windmills in Germany in 1895.  He cited his source was the book Mühlenflügel und Wasserrad. 
Matthias Heymann in his magisterial history of German wind energy noted that there was at most 20,000 windmills in Germany around 1880.
Heymann provides an extensive footnote with his sources. He first mentions Jannis Notebaart and Wolfgang Frode who cite 19,900 windmills in Germany in 1882.[4,5] Bilau reported that as many as 22,000 had been in use. German statistical reports from 1882 report that 18,901 windmills were in use.
 Robert Gasch and Jochen Twele, eds. Wind Power Plants: Fundamentals, Design, Construction and Operation. 2nd ed. Heidelberg: Springer Verlag, 2012. p. 28.
 Mager, Johannes. Mühlenflügel und Wasserrad. Leipzig: VEB Fachbuchverlag, 1986.
 Heymann, Matthias. Die Geschichte der Windenergienutzung, 1890-1990. Campus, 1995. p. 20.
 Jannis Notebaart. Windmühlen. Reprint 2015 ed. Den Haag: De Gruyter Mouton, 1972. p. 64.
 Wolfgang Frode and Edelgard Frode. Windmühlen. Energiespender und ästhetische Architektur. Köln: Dumont, 1981. p. 79.
 Kurt Bilau. Die Windausnutzung für die Krafterzeugung. Paul Parey, 1942. p. 35.
 “Stastik des Deutschen Reiches, Neue Folge, Gewerberestastik des Deutschen Reiches, Band 6.1,” 1882.