The populace will eventually win a war of attrition if we stipulate that the world has to be livable afterwards, so you can’t just agent orange the globe.

I’m just wondering if there is a critical mass for a mob, basically, at which point it overpowers even the US military. I think it’s somewhere in the millions or tens of millions, but I have no idea how to narrow it down more.

There is no amount of civilians that beat the US military. You could gather 100% of the “ive got guns to fight the government” crowd, and they wouldnt do shit against tanks, helicopters, and jets… a mob is going to die unless there is an insurrection in the ranks of the people who control the equipment.
With the world being livable afterwards?