I'd say at least four? You want Cora because Cora's the best. She can heal and do lots of kind of magic (Wynne is also good for not dying). Then you want at least someone like me or Shale, someone who can take a lot of attention and do a lot to control people on the battlefield. Then you also want someone who can deal a lot of damage, like Zevran who is very stabbity with the knives. And you know a fourth of some kind, whatever the task at hand calls for. That's much better than walking around a prison in your underwear.