Alright, if im understanding you here, "space" = number of rooms unlocked. And this space is not shared between floors? But i can have 6 people on Floor 2 even though I only have 4 rooms unlocked. And I have all 10 rooms unlocked of floor 1 but can never get more than 6-7 people in there at once.
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You can have momentraily more people because one can head inside the dungeon, while another is heading back from the dungeon, both of these two heroes are still having the same quest, but only one of them is "blocking" the room (the one that is heading in).
The "never more than 6-7" is a bit bizzarre, are you sure that you have enough heroes of that level range? Because floor one quests are very close to the entrance they are usually done with them very fast and are spending most of their time in the tavern and waiting for equipment to craft, so essentially they never enter the dungeon all at the same time.
Otherwise it's a bug, you can check if there is a queue of low level dudes waiting at the entrance or it's a constant rotation.