The creature pool is set in the levelsxx.txt for every single creature that can come from a portal. It defines the maximum number of creatures of that kind that can join into the dungeon, regardless of the player who gets it. If the pool for bile demons is set to 10 for a specific level it means that only 10 bile demons can come through portals.
If a creature dies, it is re-added to the pool.
Code(Name of Creature in script language, number of creatures added to pool)