People ON computers: Only if you are also on one. You don't game Cross-platform, usually.
Computers, meaning AI's:
Only for Firefight and the Campaign.
Chat with our AI personalities
An approximate 8.4 million people Play Halo Reach a day
they dont have a firefight demo for halo reach soz
No
You change the play list in Multiplayer.
halo reach is one of the best yet but if you want to know more about the story go for halo combat evolved first and work your way through. :)