Let's say board contain's all monsters available and maximum outskirts count is 6 when it's already has 5 monsters in it.

Monster surge occurs, bloating out 6 monsters. First monster is added to the outskirts, reaching it's limit. Next monster triggers terror level increase and outskirts wipe. What happens with remaining 4 monsters? Are they ignored and returned to the cup? Or they added to new, clean outskirts?

inb4 Tibs: Yea, you add remaining monsters to the outskirts. You also add 2 doom tokens and roll a die, losing the game on 1.

Monsters are meant to be drawn one by one.

So the remaining monsters will be put to the Outskirts after it has been cleaned.

So, in the game with 8 players and reached monsters limit amount of monsters drawn = amount of terror level increase?

Indeed. And that is why you always play with at least two expansion boards.