There was no sure number (time zones and what not) but when I player war-games for the first time I added up the number of players in each playlist. The total was easily Half a million.
I experienced very little connection problems. Though the join in session is very bad (I’m sure it can be fixed soon) It depends on where you live, If you a Ozzy then good luck.
I remember at the midnight launch of Halo 3 there were well over a million players online. Didn’t see but a quarter of that for Halo 4 … but that could also be because people were hungry for campaign.
Last night in War Games it seemed like there were a couple hundred thousand people playing. Might have been a bit skewed as most of the US population was watching the presidential race results all night(IMO).
Halo 3 counted all players online I believe, Halo 4 was just counting all the players in a matchmaking game. I know most of my friends were playing the campaign so its safe to say that there were as many as 1 million people on.