[250.50]. If you said "separated," you're wrong.
All grounding electrodes must be bonded together. And actually, it
isn't possible to "separate" electrodes. Read IEEE-142 for a
comprehensive explanation. Before doing that, review Kirchoff's Law
of Parallel Circuits and look at soil resistivity data.
[250.52(A)]. There are actually eight: metal
underground water pipe, metal frame of the structure,
concrete-encased electrode, ground ring, rod and pipe electrode,
other Listed electrodes, and other local metal underground systems
[250.52(B)]. Metal underground piping systems.
However, these must be bonded to the grounding electrode system to
[250.53(B)]. Six feet, assuming 10 foot rods. The
reason is shell theory, which basically says that if the rods as the
rods get closer than a certain distance they cancel each other out.
While the NEC states six feet as a minimum, longer is better. So,
for example, if you drive a second rod and you have 35 feet until
you reach the end of the wall you are working near it's a good
design practice to go 35 feet between rods. The extra copper in the
ground, if buried properly, will serve as an additional electrode.