I don't know, but the number required to truly get the job done, would be staggering. Frankly, it would take the combined might of all the military forces of the western world to do it.
The casualties, mutinies, and defections on the invaders' side would be horrendous, and in the end, all you'd have is a demolished country from coast to coast.
The American military has been tied down in Afghanistan for over a decade by a bunch of 7th century towelheads with AKs and IEDs. Can you even imagine the level of difficulty with prosecuting a shooting war against a culturally advanced, heavily armed, and educated patriot population?
Unless George Soros becomes Emperor of The World sometime soon, nothing remotely like that scenario is even possible. Conquer the American people by force? It's a leftist pipe dream, nothing more.
Just last week a high school in Michigan was evacuated because of a man wearing camo walked into a school.
It turned out to be an airman from the nearby Selfridge Air base dropping in to say hi to his old guidance counselor. The guy was unarmed and no threat.
I think I heard that during the Vietnam war, they did some figuring. In an open war we would need a 3:1 advantage in soldiers to win If it went guerrilla, we would need a 15:1 advantage. We had what, half a million troops there at one time? The VC were never too high tech, but they caused a lot of problems for our troops, and after 12 years we were gone and they were still there. The only reason I can think of for the govt buying all that ammo is to keep someone else from buying it.