If a FIRE and a WIND monster are on the field, you get both effects.
If there are 2 FIRE monsters on the field, Element Dragon’s ATK is 2000 (it only gets the +500 ATK bonus once, no matter how many FIRE monsters are on the field).
If there is only 1 WIND monster on the field, and you attack and destroy it with “Element Dragon”, you cannot attack again because there is no longer a WIND monster on the field so “Element Dragon” no longer has that ability.
Monsters that have their Attribute changed with “DNA Transplant” or “Scroll of Bewitchment” are included for “Element Dragon”’s effect.
If there are no face-up FIRE monsters on the field, and “Element Dragon” attacks a face-down FIRE monster, “Element Dragon”’s effect that increases its ATK by 500 points is applied before damage calculation because it is a Continuous Effect.