Strength of Armies at the Armistice, 1918
These numbers include figures for the fronts in France (west), Italy (south), Balkans and Russia (east), a total of 15,822,135. Let's round that to about 16 million men under arms when the war ended. That's a lot to demobilize and re-integrate into civilian live. The Near East is not included here.
Source: Great Britain. War Office. Statistics of the Military Effort of the British Empire during the Great War (London, 1922)