I don't have my code book handy so correct me if I'm wrong. I'm going from memory.
1. 1 electrode required. If available metal water pipe, metal building frame, encased electrode and grnd rings will all be used.
If metal water pipe is used a made electrode is also required. If the ground rod is the only available electrode it must have resistance of 25 ohms or you must add an additional rod within 6 ft. That's the nec requirement as I recall, but many times the local utility provider will dictate.
2. The requirement is 1 place only at the service only(with a couple of exceptions), if the disconnect is part of the CT can some will argue it's the same. I do know the logic behind not putting it in the meter portion is that the utility co will apply seals to that section rendering accessability difficult.
3. As long as it meets any grnd/bonding requirements of the situation.
4. No, Service entrance conduits require it.
In other locations it is basically required only on concentric/eccentric ko's for 250V and above.
5.Can't recall
6.In your follow up, you said THHN. A service mast is a wet location. THHN is dry/damp loc.
It should be THW or THWN.
For your question ? are you asking about derating for # of conductors?
