This mostly depends on what source you decide to look at and what are all the casualties you decide to take into account when talking about casualties due to World War I.
It is estimated that the total bodycount due to the war was around 38 million casualties. This is calculated together with people for who we know died, people who went missing, or people who were wounded. Out of the 38 million casualties, there were around 17 million dead and around 20 million wounded people. This really tells us about how big the scale of this conflict was.