How many people total would have died in a US invasion of the Japanese islands in 1945-46?

[Amishman]

13. read about the planning done for Operation Downfall, total deaths could easily top 1 million

The Japanese combatant deaths alone could have topped one million. The Japanese were recruiting and arming militias, with 2 million people recruited into the Volunteer Fighting Corps and were actively being trained to fight a war of attrition against the invaders. https://en.wikipedia.org/wiki/Volunteer_Fighting_Corps

The Japanese were estimated to have between 5000 and 10000 kamikazi planes available by the time an invasion force would be ready.

The wikipedia article has a number of different casualty estimates from the invasion planning, the highest of them being:

A study done for Secretary of War Henry Stimson's staff by William Shockley estimated that conquering Japan would cost 1.7–4 million American casualties, including 400,000–800,000 fatalities, and five to ten million Japanese fatalities. The key assumption was large-scale participation by civilians in the defense of Japan. https://en.wikipedia.org/wiki/Operation_Downfall