The Russian Imperial Army was unable to equip all its drafted men with rifles early 1915 - and purchases and deliveries of British and Japanese rifles were made (remaining rifles in stock were later sold at high prices to the desperate Spanish Republic 1936-38). By December 1914, the Russian Imperial Army had roughly 6,5 million men in uniform, but only 4,5 million rifles to equip them with. At the end of 1915, they had 2 million men at the front, but only about 1,2 million rifles to equip them with. However, by that time the Russian production had picked up, reaching about 1,3 million rifles per year, and there were 2 million rifles in Russian ports or on their way from Britain and Japan.

All countries fighting in ww1 suffered huge problems trying to supply their forces and equip men raised to replace the extreme casualties suffered by the pre-war armies during the first year of the war as well as the expansion of their armed forces. This was especially bad for Russia, and especially December 1914-December 1915. It took time for industry to change to war production, and the lack of nitrates (in the Central Powers due to the blockade, in the Entente due to difficulties in increasing shipment of guano) hampered production of ammunition. The British had to purchase Canadian and US rifles and took rifles from the Indian army as a stop-gap measure. However, the Russian Imperial Army was the only army to send uniformed but unarmed men into combat, expecting them to arm themselves on the battlefield late 1914 to late 1915.

So the Russians could not equip more men, as they were already unable to equip the men they did mobilize. Mobilizing more would not add to their combat power, but would draw men off from their industries and above all the countryside, where many worked as farm hands and in similar roles - in 1914, half of the Russian plows were wooden and most agriculture was conducted communaly, which paired with the lack of mechanisation (bot horse-drawn machinery and tractors) meand that large amounts of labour was needed to bring in the harvests. Mobilizing too many soldiers could endanger the food supply.

Sources:

Brusilov Offensive, by Timothy Dowling.

Race to the Front, by Kevin D Stubbs.