Depends? That's the best answer I can give. The number will vary a lot depending on the amount of information you need to send, the number of people playing the trivia game at the exact same time, the power in each server. I can give you a bit of a ball park figure: a Go Native! (MMO Trivia game) server can take care of 2 people per mb of ram. Give or Take. The amount of data we send between client and server is very tiny.
42. Seriously though, you will never need to worry about that problem. You will never have a game with 500 million people playing it.
Half a billion at same time! You will need 'The Internet' for your servers. Might as well optimise ASCII and drop the letter A, that should save a few countries worth of computers... Edit: wow even a billion isn't as much as it used to be (at least around here!) http://oxforddictionaries.com/page/howmanybillion
I don't want to doubt your question, but there are no games out there being played by this number of people at anyone time. Even the biggest game brands only have millions on at one time. So maybe you could give more ideas for your reasoning why you think you need 500 million first.
It pains me to say it given the damage that Americans have done to our language, but they were right and we were wrong.
Not on at the exact same time. The most active playerbase I can think of is Zynga Poker - they display the numbers and it's always between 200,000 and 350,000
To basically echo an earlier comment, this depends on exactly what sort of "support" the servers are responsible for (simply storing high score submissions? game scenario and live player hosting a-la MMOs?), how the information is packaged, the quality of the servers themselves, etc. Is this a question because you intend to construct a game that will attract this many users (sell me stock!), or professional curiosity for, say, resource estimates if Angry Birds were to host a global leader board?
Who knows how many but Facebook has some of those numbers I think however thats a different story of course