As everyone knows(?), Martians have four fingers (or three fingers and an opposable thumb if you want to be pedantic) or toes at the end of each appendage. So I can't think of any reason why they would have hit upon a decimal system of counting. As for humans, two handsfull seems to be a good unit of measure (stop sniggering at the back!), so an octal system of numbering would make perfect sense. To the dyscalculic amongst us, that means that Martians count on their fingers as follows: 1, 2, 3, 4, 5, 6, 7, 10, and then 11, 12, 13, 14, 15, 16, 17, 20 after removing shoes and socks.
I can therefore imagine their military units being based on multiples of 8, giving theoretical values as follows:
Somewhat bizarrely, we end up with the nice octal round numbers used by my Martians being the same as appeared in classical Greek tactical manuals! Maybe there really was something extraterrestrial about Altantis after all?
OK that worked out pretty well, and gives comparable unit sizes to terran counterparts.
Next stop ... Calendars and Time. I'm betting the answer is 42, but we'll see.
Real World Footnote:
Apparently the Yuki language in California and the Pamean languages in Mexico use octal number systems because the speakers count using the spaces between their fingers rather than the fingers themselves. Ain't Wikipedia a wonderful thing?