At the start of the battle
Upon entering the battle, you have a few turns with no enemies in order to position your survivors.

During the battle
Throughout the battle, there are red arrows on the map, which indicate the direction from which the upcoming wave of walkers will spawn. Walkers don't generally spawn from the same direction on two consecutive waves, but there will be waves every now and then coming in from all four sides of the map.

At the bottom of the screen, you can see the amount and type of walkers that will spawn in the upcoming wave.

Note: Unlike other game modes, Last Stand doesn't have threat mechanics, meaning that your survivors cannot generate nor reduce threat