6, 8, 10 Or 12

Poul Anderson, After Doomsday.

If intelligent beings with arms have an even number of arms and also have the same number of fingers at the end of each arm, then they have an even number of fingers. (Which fictional aliens have an odd number of arms?) Furthermore, if such beings learn to count on their fingers, then their base number is going be even.

"[Monwaingi] arms, thinner and weaker than human, ended in hands whose three fingers, four-jointed and mutually opposed, were surprisingly dextrous." (CHAPTER ONE, p. 12)

"Biped, about as tall as [Donnan] was, with powerful arms ending in regular five-fingered hands, [the Vorlakka] were placental mammals and bio-chemically very similar to men." (CHAPTER FIVE, p. 55)

"The Monwaingi based their arithmetic on six. But this was a Vorlakka ship, whose ten-fingered builders used a decimal system like Earth's." (CHAPTER SIX, p. 65)

"[Kandemirian] hands were also humanoid, in spite of having six fingers and jet-black nails." (CHAPTER SEVEN, p.74)

On the planet, Zatlokopa:

"Sigrid looked at her watch. By now she was used to the time units, eight-based number system and revolving clock faces employed here." (CHAPTER EIGHT, p. 82)

Revolving clock faces?

"'...Kandemirian numbers are based on twelve...'" (CHAPTER NINE, p. 93)

We are not told what the Zatlokopans look like but we infer that they have two four-fingered hands. Anderson carefully correlates the number of Monwaingi, Vorlakka and Kandemirian fingers with those species' base numbers. This matters later.