The correct answer is - around 19.7 million.
The World War I was one of the worst things that happened to the humanity. The casualties in this war are estimated somewhere around 40 million. Of those 40 million, around 19.7 million were deaths. 9.7 million deaths were of military personnel, while around 10 million deaths were of civilians, innocent people that had nothing to do with the war and the interest of the political elites.
The World War I was waged between the Central Powers consisted of Germany, Austria-Hungary, Bulgaria, and Turkey, and the Allied Powers consisted of France, the British Empire, and Russia. The end result was a win of the Allied Powers, lot of restrictions put on Germany, the fall of Austria-Hungary.
