Erickson gives a figure of around 36,000 reported prisoners of the Russians.Adding unreported cases would appear to support the overall figure of 50,000 captured by the Russians.
Erickson in Ordered to Die,page 189 gives their fate:
Overall only 9,010 of the 50,000 Turkish POWS in Russia returned home.By July 21 1918,the Ottoman government knew that 1,457 officers and 17,715 who had been captured by the Russians had been reported alive in Russian POW camps.That summer these men began to make their way by train,and then by foot or boat,home to Turkey...of these known prisoners,only 2,260 returned to Turkey.However an additional 6,750 unaccounted for prisoners and 2,250 civilians returned from Russian camps.These returnees reported that 15 per cent of their number had been massacred while in convoy en route.The fate of the others remain unknown.