Eight Allied nations signed Japan’s formal surrender instrument after MacArthur and Nimitz.
The Japanese Instrument of Surrender was signed aboard USS Missouri on September 2, 1945. General Douglas MacArthur signed first for the Allied powers, and Fleet Admiral Chester Nimitz signed for the United States. Representatives of eight other Allied nations then added their signatures: China, the United Kingdom, the Soviet Union, Australia, Canada, France, the Netherlands, and New Zealand.
This count can cause confusion because the full ceremony involved more than eight Allied signatories. MacArthur and Nimitz signed before the representatives of the eight other countries, so the number in the question refers specifically to those additional Allied nations.
The Japanese side had two signatories: Foreign Minister Mamoru Shigemitsu for the Japanese government and General Yoshijirō Umezu for Imperial General Headquarters. The document therefore recorded both Japan’s civilian and military surrender authorities.
The ceremony was internationally significant because the surrender was accepted on behalf of a broad coalition, not only by the United States. The signing ended the global conflict in the Pacific and began the Allied occupation and reconstruction of Japan.