In May 2013, it was reported that there were an estimated 17,625 nuclear warheads in the arsenal of governments worldwide. The estimated number of warheads was down from the 19,000 at the beginning of 2012.
Russia has about 8,500 nuclear warheads in its arsenal, and the United States has 7,700. China is the only country in the world that was building up its nuclear arsenal in 2013. It was estimated that China increased its warheads by 10 in 2012, giving the country a total of about 250 nuclear warheads.
Source: Lily Kuo, “China is the only nuclear weapon state expanding its nuclear arsenal,” Quartz, June 3, 2013.