If you control more than one Fire Servant, the effects are cumulative. Two such effects will cause damage from red instant or sorcery spells you control to be multiplied by four; three such effects will cause damage from red instant or sorcery spells you control to be multiplied by eight.
If multiple effects modify how damage will be dealt, the player who would be dealt damage or the controller of the permanent that would be dealt damage chooses the order to apply the effects. For example, Mending Hands says, “Prevent the next 4 damage that would be dealt to any target this turn” and Lava Axe is a red sorcery that says “Lava Axe deals 5 damage to target player or planeswalker.” Suppose a Lava Axe controlled by a player who controls Fire Servant would deal 5 damage to a player who has cast Mending Hands targeting themselves. The player who would be dealt damage can either (a) prevent 4 damage first and then let Fire Servant’s effect double the remaining 1 damage, taking 2 damage, or (b) double the damage to 10 and then prevent 4 damage, taking 6 damage.
Fire Servant’s ability applies no matter who or what the damage would be dealt to: a creature, a player, or a planeswalker.