Nine Allied nations signed Japan’s surrender instrument in addition to Japan.
The Allied signatories were the United States, China, the United Kingdom, the Soviet Union, Australia, Canada, France, the Netherlands, and New Zealand. Their representatives signed aboard USS Missouri in Tokyo Bay on September 2, 1945.
The United States signed through General Douglas MacArthur and Fleet Admiral Chester Nimitz. The other Allied countries were represented by senior military or diplomatic officials, including General Xu Yongchang for China and General Kuzma Derevyanko for the Soviet Union.
Japan had two principal signatories: Foreign Minister Mamoru Shigemitsu signed for the government, and General Yoshijirō Umezu signed for the armed forces. Counting Japan alongside the nine Allied nations gives ten countries represented by signatories, but the question asks only for the Allied total.