Hi. 'find me a source': Google is your friend.
I can understand if you don't want to learn about this stuff. You sound young, or naively gloaty at least. If you think talking about 4 people is relevant then prepare for a big shock. It's really a horrific story though. I learned about it until I couldn't stand it anymore...all that stuff about central America in the 80s is so depressing. But no more than e.g. 800,000 (!) bombing raids on Laos in 1964-73, dropping more bombs than in the whole of WWII. Understandably it's hard to get precise figures, or any kind of figure, for killing on such a huge scale. You guys don't bother counting these days. Do people born with birth defects from chemical weapons count? Killed with unexploded bombs after a 'war' - 20,000 in Laos alone since 1973. ... Did you hear they were using uranium bullets in the latest Iraq slaughter?! I wonder how long that will cause 'problems'. I guess I first learnt about most of this from Chomsky's earlier political books, and following up his references and reading elsewhere, until it was all too depressing and I had to stop. He wasn't making it up. The way he straight-facedly reports the grotesque pronouncements of officials, letting the facts condemn themselves, is not something forgettable.
I found this source[0] saying 20 million after a moment's googling. (And a minute checking the site didn't seem obviously insane.) I'm no expert at all. I didn't think "millions" was particularly controversial. I don't think I've seen a total figure estimate before, but from the stuff I've read about Philippines, Vietnam, Cambodia, Laos, central America, Iraq x2 etc etc, I thought "millions" was pretty inarguable. Just read about all the places the U.S. has invaded/bombed/'interfered with' in the 20th C. You'll be doing well to make it halfway. Often because they were getting too democratic, not doing what the U.S. wanted, the "threat of a good example", that's the depressing thing. Then cover the whole thing up with propaganda so the U.S. public thinks it was all in a good cause. Pretty sickening.
Do you only count 'domestic' human rights abuses? And 'non enemy combatants'? So if they fight back, their human rights aren't abusable or something. (Or they just slaughter whole villages and say they were enemy soldiers. See wikipedia page on Vietnam casualties) And I guess 'directly caused' means if the U.S. gets rids of democracy and installs a dictator (e.g. Iran), and sells them weapons, like they did so many times in the 20th C, (and probably graduates of U.S. torture school) well, anything bad that happens later isn't 'directly caused' by the U.S.?
Anyway. Oh also Scott Noble's documentaries are available on his site[1], some covering aspects of this subject. I think they're excellent, also very depressing, although they feature some very inspiring people.
[0] https://www.globalresearch.ca/us-has-killed-more-than-20-mil...
[1] http://metanoia-films.org/films/