How many of the Avengers knew about the effect of the Thanos snap? Gamora seemed to know coz she talks about Thanos wanting to wipe half the population. Thor didn't seem to have any idea even after the snap happened as was the case with the Avengers in the Wakanda battle and on Titan. Strange knew it because he saw into the future. Fury seemed like he had an idea about it. And that's it.

In that case, (quoting from the Endgame trailer) who told Black Widow about the snap. She says, *‘Thanos did exactly what he intended to do, he wiped out 50% of the world's population.’*  It even seems to suggest that whoever she was talking to also knew about the snap? 