Has there been any information or screen shots  showing that you can have more than 2 people in battle at one time.  Also has there been any confirmation on which of the characters other than (Cartman, Kyle, Kenny, and Stan) join your party?

I think in going with the Paper Mario motif, there will be only two. Would like up to three, though.

i suspect it'll be 3, maybe even 4 based on the first quest 3+ you=4. I remember an interview where they said they were adding more and more content to the game at the time it was 800 pages long, so i can't see that being just for one or two characters. 

2 charachters in combat and field

 

Buddies/Companions are

 

Cartman

Kenny

Butters

Stan

Kyle

Jimmy

 

I'm sure there will be some way to switch.