Eight Allied nations besides the United States signed Japan’s Instrument of Surrender in 1945.
After Douglas MacArthur signed for the Allied powers and Chester Nimitz signed for the United States, representatives of China, the United Kingdom, the Soviet Union, Australia, Canada, France, the Netherlands, and New Zealand signed the document.
That makes nine Allied national representatives in the sequence if the United States is included. The question asks for the number besides the United States, so the correct total is eight. This counting distinction is the main reason the figure can be misreported.
The ceremony took place aboard USS Missouri in Tokyo Bay on September 2, 1945. The signatories represented the major Allied powers that had fought Japan or, in the case of some governments, participated in the broader Allied war effort.