Earthing and Grounding

By experiment. Local ground conditions, not least of which is the height of the local water table, will determine this.

last answer a little of the mark.

Size the electrodes according to NEC 250. (The GEC also), then,

It will not be a calculation but a measurement. The NEC states that at 25 ohms between the service and grounding electrode is when you need a second rod on one service. They don't provide any advice if this doesn't solve the problem though!

If it's a SDS, then it will need its own ground rod if put into interactivity with the grid or is placed in operation for a building of any type. If it the equipment is fed or backfeeding service equipment on a separate foundation then the service is located, it will need a grounding electrode. Very general rule: every separate foundation needs an electrode.

Any additional grounding electrodes must be directly bonded to each other.

My answer is a little oversimplified.

Ideally, zero ohms is what you want on the grounding electrode connections. "a low impedance path". Under 5 ohms is a more realistic target.

Use concrete encased electrodes. Ufers. This will improve this performance.

Consult a licensed electrician.