I personally prefer to split Chest/tris and back/bis, because they are complementary muscle groups. Chest and tris are both "push" movements, whereas back and biceps are both "pull" movements. For example, depending on where you arms are on a the barbell for a bench press, you will work your triceps just as much if not more than your chest (but potentially wreck your wrists too so be careful!). If you do your split like this, you won't have to worry about overtraining a muscle group if you do these days back to back.
Try doing pushups with your index fingers and your thumbs touching: your chest and triceps will be feeling it. If you do a close-grip pull-up, your lats will be on fire just as much as your biceps (Ok, maybe not as much as your biceps, but your lats are working hard, too!).
As for number of exercises, I believe that pairing the right exercises with the right amount of weight and paying attention to tension in your muscles is more important than "the number of different exercises" you do. If you only do two but do them with intensity, it may be more effective for you than doing 6 different exercises all half-assedly.