We've seen a lobby in the earlier footage of the game. In that lobby there were 3 players. Bungie has said that your fireteam will be 3 players big. We don't know if you can play co-op with more than 3 players, and we don't know the size of multi-player. The 'shared world' thing, I believe relates to finding people out in the wild and public events. You're not sharing your world with other people until you are matchmade with them. It's not like an MMO. It's like Halo 3's matchmaking except it is going on in the background, while you're playing. So, say you're on Mars, in a particular area, Bungie can match you and other players together. Perhaps there is a fireteam of 2 fighting some bad guys just over the hill (in their world). You are of similar skill, maybe you're doing the same mission, it's going to matchmake you together. So now you're sharing a world. It's not that you're on the same server like in an MMO, it's that you were matchmade to play together, and you can, or you can go your seperate ways. Bungie still hasn't clarified how all of this works.. so this is some speculation on my hand. More speculation on my hand, and more to your original question. No, I don't think there will be as many players as 50-60. Bungie has been great at making maps for the 2-16 player range, and I think you'll continue to see those numbers in these games. I know there is the allure of massive battles, and Bungie will be supplying that in campaign and strikes/raids, public events or whatever they have, but not in PvP. The max I could see in PvP is 24. That would be 4 fireteams of 3, versus 4 fireteams of 3 (12 v 12). Fireteams working similar to how Invasion worked. You can spawn on your fireteam, or at a spawn location, or randomly spawn, who knows. But yeah, I wouldn't expect to see the large numbers like you do in Battlefield. Bungie maps are much better designed, they are meant to have you in the action, they are much better thought out. Something a huge map doesn't do very well. As for actual lobby size, I don't know if they'd allow the max number of players in one lobby, or just your fireteam. We'll all have this answered sooner or later, probably at the beta or close to the beta.

From what I understand there isn't a lobby. Its a shared world.