on the halo 3 multiplayer campaign feature there can be 2 or 4 players (accept online)
Up to two players can play in the Co-Op Campaign mode on Halo: Combat Evolved.
you do apparently have an illegal version of Halo. get the legal version of halo to play the campaign.
Over XBOX Live, there can be at the most four people playing one campaign session. On a single XBOX 360, only two people can play at the maximum.
no halo "campaign" is a single player game
What do you mean "How do you play solo campaign on Halo 3?" Just go to the Main Menu and select campaign and start Solo Campaign.
Yes, you play as the Arbiter in certain halo 2 campaign levels. I believe there are mods too. You can also select Elita as a character model in halo 2 multiplayer.
You can invite people to play the campaign with you over xbox live.
The number of people who play Halo is not known.
Only in Online Campaign, Not Local Play
Campaign is 9 missions you have to play through as Master Chief.
An approximate 8.4 million people Play Halo Reach a day
The only time you can play against an NPC is during campaign, otherwise you have to fight your friends or other people.