"שניים עשר קופים."
It's the way Semitic languages construct the genitive, that is, it's the way Semitic languages indicate possession. English can use an apostrophe or "of," for instance. "The boy's book" // ספר הילד. Consider googling "Hebrew construct state" and getting a short grammar of biblical Hebrew and also modern Israeli Hebrew to accompany your study.
I think that properly it must be one yod. But I'm not sure. I was somewhat surprised to see it should be spelled with one, though in retrospect it makes sense. Certainly two yods would look correct to almost everyone. I also "blind tested" my wife, who has excellent sense for this stuff, what's correct; she hesitated a lot, and guessed wrongly.