KYIV, Ukraine — The head of the Russian private army Wagner says his force lost more than 20,000 men in the drawn-out battle for Bakhmut. He said about half of those who died in the eastern Ukrainian city were Russian convicts recruited to fight in the 15-month-old war. The figure stood in stark contrast to the widely disputed claims from Moscow that just over 6,000 of its troops were killed in the war as of January.