In the adventure map creatures appear in wandering creature stacks. Without Visions, a player can not see exactly how many creatures there are, but they can see a rough estimation as seen in the table below.
On the battlefield, Creatures appear in stacks. Stacks take their turns according to initiative and can move and/or attack on their turn. When a stack takes damage, the 'top' unit of the stack loses health, and when it reaches 0 health, reducing the size of the stack by one and revealing the next-top creature, which starts with full health (unless the damage taken exceeds the current health of the previous top creature, in which case the new top creatures loses health accordingly). A stack deals damage according to the number of creatures in it. When the last creature in a stack dies, it leaves a corpse on the battlefield which can be the target of spells/abilities like resurrection. All of the creatures that were in a stack at the start of the battle are in the corpse (rather than, say, the stack leaving small corpses behind every time any of the creatures in it die).