In all 18.6 million men served in the Russian Army during the war, more than a tenth of the total population and from all ethnic groups.

From The Long Shadow: The Legacies of the Great War in the Twentieth Century by David Reynolds.  Page 7.

A similar story may be told of the Russian Empire. In 1914 the Romanov dynasty ruled about 170 million people (nearly four times the UK population) across one-sixth of the world’s landmass, yet fewer than half of them were ethnic Russian. The regime failed to create a sense of overall imperial identity, or even a sense of nation among the core Russian population. Belated and often brutal attempts by the last two tsars, Alexander III and his son Nicholas II, to impose Russian language and Orthodox religion served only to inflame nationalist sensitivities. Then, after the abortive revolution of 1905, the government reversed itself with halfhearted political concessions that gave national groups a voice in the new parliament, the Duma. Particularly sensitive was the issue of Poland—a historic state that had been partitioned since 1772 and was now largely under Russian control. Yet despite a few anti-draft riots, war mobilization in 1914 proceeded relatively smoothly, with nearly four million men conscripted on schedule. In all 18.6 million men served in the Russian Army during the war, more than a tenth of the total population and from all ethnic groups.