Multiple Embermaw Hellions are cumulative. If you control two of them, you’ll add 2 to the damage dealt by another red source you control. In this case, you would also add 1 to the damage dealt by either Embermaw Hellion.

Embermaw Hellion’s last ability doesn’t cause Embermaw Hellion to deal damage; it affects the amount of damage dealt by the original red source.

If the player or permanent being dealt damage is also affected by a damage prevention effect, that player or the controller of that permanent can apply that effect and Embermaw Hellion’s effect in any order. If all of the damage is prevented, Embermaw Hellion’s effect can’t apply to it.