I am not very familiar with USA power-grounding regulations and they
probably vary from state to state. But I am under the impression that in
some simple circumstances, perhaps domestic, a single ground rod is
considered adequate for safety purposes.

Under similar circumstances a maximum ground electrode resistance ( whatever
its construction ) of 50 ohms is specified.

A 'standard' ground rod is 8 feet long and 1 inch in diameter.

An average soil resistivity is 500 ohm-metres ( = 2 mS ). See Eznec?

The calculated resistance of a single standard rod in average soil is 194
ohms. ( I'm sure the calculating formula can be found somewhere on the IEEE
shelves. There ought to be greater use made of it. )

Now I don't expect the alarm bells to be rung from Washington to LA. But
isn't there a serious inconsistency somewhere?

As a matter of interest, the resistances to ground of 2, 3 and 4 rods in
parallel, spaced 6 feet apart in average soil, are as follows -

2 Rods = 113 ohms
3 Rods = 86 ohms
4 Rods = 70 ohms

all of which exceed the specified maximum of 50 ohms.

Quite a low average soil resistivity of about 130 ohms is needed to bring 50
percent of installations within specification.

If my starting data is wildly adrift then disregard my waffle.
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Reg, G4FGQ