For my culture, I'm basing things on multiples of 6 and 12 - or, at the very least, factors of 12. So they may have 48 alphabetical characters, each representing the 48 "sounds" that they make. 12 punctuation characters, etc. It's mystical, so they may have created systems with 64 spots, but only have 59 of them in use as they search for the missing 5.

Etc. etc.