There is no definitive number. Exact casualty figures from any war is a relatively recent phenomenon, and the First World War is definitely not a recent one. Both sides sent millions of men into that meat grinder and the battlefields of France, Belgium and Germany remain dotted with crosses which state, "A Soldier of the Great War, Known only unto God" We'll never know, so you basically have to read the estimates and support only those that seem reasonable to you.
Dean.