I believe the quests were written before they finalized the monster count, which is why there are a few quests that use too many monsters.
I've heard that, too. As a rule of thumb, if a single room all by itself asks you for more of a given figure than the game provides, it's probably a typo. All such mistakes I'm aware of were errata'd to say "use a replacement figure as a stand in for this one."
If you run out of figures of a certain type because you've been spawning too many or because the heroes have been opening new doors at a ridiculous pace, then you're looking at the whole "can't place more than you have" situation. The good news is if you've really got that many monsters on the board, the heroes are probably going to have their work cut out for them.