The United States now enters the phase of mass executions.

[BrianSoddingBoru4]

Based on your metric, the Dakota executions mentioned above stand as the ONLY mass execution in US history. Nine judicial killings over two months don’t even crack the top ten.

Again, it depends on one's criteria:

Los Angeles Times -- Graves Recall Executed German POWs

Groups of 5 and 7 were executed by the United States on the same day.

Amnesty International condemned the "mass execution" of 21 individuals:

Iraq: Mass execution of 21 individuals is an outrage

And, so, I do not think that I am too far off the mark here.

But you seem to keep shifting the criteria you want to use.

Boru