Just as the title says, and it doesn’t even have to be in the same location if you want to include a simulcast event.
Well, the first number to beat is 25,272, the largest group of carol singers according to the Guinness Book of World Records.
I can’t speak to simultaneous singing but “We Are the World” was played simultaneously on more than 8,000 radio stations worldwide in 1985.
Looking for greatest number of singers, not listeners-sorry.
Given that there are about 200 million people in Brazil. That’s probably close to the ammount of people that sang along with the national anthem before the 2014 semi final of the world cup.
Honestly, I’m in the Netherlands and when the national team plays in a world cup quarter final/ semi final at least 10 million of us 17 million will watch the game. Estimating that 1 in 10 will sing along with the anthem is a low estimate. So that would be around 1 million. For the fricking Netherlands!
Almost everyone in the world is singing a masterful arrangement of John Cage’s 4’33" right now.
Except many of them are singing it in the wrong key.
Everyone has moved on I guess, but I still think a “song” is something that is sung, and instrumentals (or experimental silence) are “pieces” or maybe “tracks” or even “instrumentals”.
Hands Across America: A charity event staged on Sunday, May 25, 1986 in which approximately 6.5 million people held hands in a human chain for fifteen minutes along a path across the contiguous United States. At 3 p.m Eastern Time, the theme song “Hands Across America” was played simultaneously on hundreds of radio stations across the country. Even if only 50% of the people in line sang along, that’s a helluva lot of people.
I’m just waiting for someone to send me the lyrics. 
I’ll grant that 4’33" is traditionally not played, rather than not sung. But if people can “sing” Imperial March for Star Wars as “Bum, bum, bum, bum-ba-dum, bum-ba-dum” even though it is an instrumental, then they can sing 4’33" as “…”.
In what key-Silent “E”?