Getting ready for monster book...

I think the reactor that summons the King Goblins disappears once the monster is out. They come back when you finish killing King Goblin though xD

Just in case it matters, I think monster limit for maps is 50.

Monster limit varies from map to map, but yeah, for most areas 50 is a good guideline.

You can actually summon more than one of a goblin at a time, however the reactor becomes slower and slower after you've summoned a few. When you start getting 3-4 out there, it becomes tough to drop more summoning items too :p