Comment by kryptiskt

I once worked at a bookmaker with millions of customers where they had extreme reliability demands during very popular events, since any bets that the punters couldn't place during live betting was lost money. They had one massive server with a big pipe and an identical hot spare in another location. It worked very well for them. That was a decade ago, you can get even bigger servers these days, there's a lot of room for scaling without needing to go beyond one machine.