Per the NEC, the rod must be a minimum of 5/8" in diameter, {250.52(A)(5)(b)}you would never need more than two rods and a #6 for the GEC.
If you are trying to meet a performance spec, you would need to install as many as it takes to get to the resistance number set by the spec and a larger conductor would help.
Roger
[This message has been edited by Roger (edited 11-19-2005).]