Kickstarter specified 5 worlds at 175,000 pounds; I'm assuming that the large amount of money they've garnered for the project from kickstarter and most likely other sources, allowed them the time and resources to squeeze in just that little bit extra. That would give 25 pagies per world and 20 pagies from hivory towers, giving 170 pagies. honestly I can't even imagine how big these worlds must be.
By the way, for what the 6th mystery world might be, I've read and heard quite a few times that in banjo-tooie, cauldron keep where you fight grunty was actually meant to be it's entire own full-fledged world, which is why tooie had 8 worlds instead of kazooie's 9. Given their statements in the past and what I've garnered about the development of kazooie and tooie, it's quite possible they took the chance to resurrect that idea. Moon Base, anyone?