I'm gonna paste this in because pterry was a god (small G), and there should be no point ever where a reference to him shall go unnoticed.

‘Everyone knows trolls can’t even count up to four!’

(In fact, trolls traditionally count like this: one, two, three, many, and people assume this means they can have no grasp of higher numbers. They don’t realise that many can BE a number. As in: one, two, three, many, many-one, many-two, many-three, many many, many-many-one, many-many-two, many-many-three, many many many, many-many-many-one, many-many-many-two, many-many-three, LOTS.)

I take "lots" in that quote to be 100 (base 4) or 16 (base 10). Then Jupiters 79 moons would be counted as "oodles-many-many-many-three" -- assuming "oodles" as 1000 (base 4) or 64 (base 10).

Presumably because trolls only have 4 fingers on each hand?

There are finger-based counting systems with more than one number per finger. Our average of 10 fingers can handle a lot more than 10 numbers. https://en.wikipedia.org/wiki/Finger-counting

And less than a digit per err digit (thumbs included)

If I recall there was a Native American base 8 counting system which utilized the space between fingers holding sticks or strings as place holders.

Made for a handy octal abacus.